The Marsh Cave. Bring tons of antidotes (and potions too, but that's a given for any dungeon in Final Fantasy — this is just the dungeon that first suggests it). There's plenty of Goddamned Bats, but also Goddamned Spiders and Goddamned Undead. And those Goddamned Cthulhumanoid Wizards. If someone dies, you have to go all the way back to town to revive them. And the way back is looooooooong. And you'll most likely get poisoned along the way.
While the poison is very annoying, the Undead frequently paralyze your team. And being paralyzed means that character is now useless, and it stays that way for a long time. Add to this that most of your characters are going to die from a few hits, and that you'll be exhausted from the damned Squid-Wizard fight on your way out.
Oh, and the slimes and oozes. Which, due to their defense, were pretty much impossible to kill without using magic spells, which you have precious few uses of. Ask five people, and they'll come up with ten reasons why the Marsh Cave belongs on this list.
The Ice Cave. Disproportionally happy music: check. Birds whose touch turns you into stonenote Reversible with Gold Needles, but they're very expensive: check. Mindflayers whose touch can kill: check. Dark Wizards who always get a free turn, attack in groups and have a 50% chance of attacking with the most powerful multi-target fire spell: check. No plot significance whatsoever: check. Having to get that dumb crystal out of there anyway: check.
What makes it even worse is that in the original NES version, there were no Gold Needles. If you got turned to stone, the only way you could be cured is if your White Mage had the SOFT spell, which didn't work in battle, or Soft potions, which were only sold in one town.
And many people will go into the Ice Cave far earlier than the normal plot would otherwise suggest, as it can be used in a common Sequence Break to get the airship, and thus get the class promotion before the volcano. The only way to get through this part without being driven mad is to finish the Citadel of Trials first (which gets you a handful of infinite-use magic items), and even with that advantage, it's still the ninth level of Hell.
These two levels are also the most difficult areas in a Solo-Character Run. Petrification is an instant game-over, and being stunned is a way to sit there and watch as your character gets slowly nibbled to death.
The top floor of the Sky Palace fits the bill. While the aforementioned One-Hit KO using Sorcerors/Mindflayers appear all over the Sky Palace, they're particularly common around here, usually in swarms of seven. On top of that, the area also is home to WarMECH, the most famous Boss In Mooks Clothing in the series. About the only saving grace is that Tiamat, the boss waiting at the end of this gauntlet of evil, is not only vulnerable to a couple One-Hit KO spells, one such spell can be cast freely by using an item that's available well before you fight the boss.
The dungeon of Deist, which you have to go through twice to advance the story further.
The very last dungeon in the Souls of Rebirth scenario. Nearly every single enemy is a Demonic Spider and you can come face to face with enemies with sickeningly high rates of slamming you with bad status effects, such as the dreaded Malboros. You most likely also will have three characters who are very squishy and a fourth character who will be doing all the fighting, without severe Level Grinding.
Both the original NES version and the DS re-release has the back-to-back final dungeons Crystal Tower and Dark World. The Crystal Tower has space filling paths (of the switchback and false path variety), the most demonic of Demonic Spiders in the whole game, and an obnoxious boss at the end. When you're done with that, there's a long unskippable Cutscene, then you go to the Dark World, which has all of this PLUS a literally unbeatable boss at the very beginning, FOUR obnoxious bosses, and then the stupidly hard final boss after all of that. All of this which would be par for the course in any The Very Definitely Final Dungeon, except as soon as you enter the Crystal Tower, there are no save points. Not a one.
The Cave of Darkness is bad enough with seven floors of winding paths, hidden passages and switchbacks swarming with difficult enemies. The kicker is that all the enemies split in two if you hit them with any physical weapon other than a dark sword/katana. That means you have to use Dark Knights (whose weapons and armor aren't that good until after this dungeon) or magic users (in a game where Ethers and Elixirs are in woefully short supply) to slog through the dungeon, or none of your battles will ever end; even then, the aforementioned multiplying enemies are time-consuming to kill and give lousy XP and money. Despite all this, the Sylx Tower/Dark World sequence is still harder.
The Sealed Cave, a long level that can be hard to navigate, contains several powerful enemies including literal Goddamn Bats, and is home to That One Boss, Demon Wall. Furthermore every door in the area is a Trap Door, an enemy that spams the One-Hit KO attack "Ninth Dimension" and turns into a Manticore when it dies, which is also a strong enemy. To add insult to injury, many Trap Doors guard empty rooms, so you likely just wasted your time. There's also an Event Flag when you leave the dungeon, so you can't use spells to warp or teleport out, you have to take the long way. In some versions, you can Reflect Ninth Dimension, but your timing needs to be spot-on thanks to how it's cast, and you have to know it can be Reflected in the first place.
Square-Enix must know that the Sealed Cave is That One Level and Demon Wall is That One Boss, because in The After Years and the new Interlude chapter in the PSP port, we get to experience them again! The trip through in After Years is even harder than before because you don't have Reflect this time to stop Ninth Dimension. Fortunately, you can use Stop to incapacitate the doors outright, but this means your spellcaster is going to run out of MP like she's sprung a leak.
The Trap Doors are so dangerous, the only way to make a solo Cecil challenge manageable in the original is to use a glitch to skip the cave. Otherwise, you need to level up into the 70s to be able to kill the doors before they kill you.
The Sylph Cave and Summoned Monsters Cave/Eidolon's Passage are candidates for this too; they're the only dungeons with damage floors (forcing you to recast Float on the party every new level, or whenever a character is knocked out), and they're full of secret passages and confusing turns (got turned around? Time to recast Float again!). The enemies are no picnic either—the Eidolon's Passage has devilish enemies that can reduce the party's HP to single digits in a single move and the one-eye enemies that are fond of Instant Death; Sylph's Cave has the infamous Malboros. Even worse, one has a number of teleport pads in it that bring you deeper in; however, there's one pad near the end that will toss you right back to the World Map with no warning, forcing you to do the whole thing over again and hope you remember where the exit one was. And finally, you've got to go through them at least three times altogether (if you're strong enough to defeat both bosses in the Summoned Monster Cave in the same trip. If not, your only option is to grind and come back later).
The Cave of Magnes is another difficult level (one with a far more persistent frustration factor), especially since being forced to remove Cecil's metal equipment renders him almost useless, and he is unable to act if the player does not do so. And your mage for that segment has his MP capped at 90. Even with your party losing its best physical attackers, the monsters are still a step above those of the previous dungeon, but without their elemental weaknesses.
This is even worse in the DS version because Cecil loses the ability to use a bow and arrows. At least in other versions of the game, you could give him a bow and some Holy Arrows.
The DS version does have a set of elemental "shards" in the nearest town that Cecil can use without suffering the penalty of using metal. It's only a small upside, though; the shards are extremely weak weapons, and even against enemies weak to that element the damage is most likely going to be less than what his normal equipment would do.
The Lair of the Father in the DS remake. Every random encounter is strong enough to take out your entire party if you aren't careful. There are three Behemoth mini-bosses, which mostly just serve to drain your MP and items before you reach Bahamut. Also, there's no save points- if you die, you just wasted at least an hour. Thankfully, the behemoths don't respawn, so you can teleport out to heal after you've fought one.
Porom's Challenge Dungeon. You're on a strict time limit. You have to satisfy the arbitrary desires of random NPCs, which include wasting Porom's MP on cure spells, giving away expensive armor, fighting powerful monsters, and so forth. You're only given two characters that can surpass level 20, one of which is Porom herself, who is a White Mage and can't really fight. Your party makeup and time limit would imply that you should run from battles, but then you won't have the money to buy much of that armor you have to give the greedy NPCs, both because you can't get money if you run away and also because you randomly lose some money in equal amount to 25% of the amount you would've gotten for winning the battle. And even if you satisfy the request of every single NPC on the map and make it to the final treasure in time, you're still not guaranteed to get anything worthwhile for it.
Rydia's Challenge Dungeon. The dungeon is a maze, though not a very complex one. What makes it challenging is that it is filled with Trap Doors and Demon Walls — yes, the same ones from the Sealed Cave, exactly as strong as before. You need to level grind Rydia to learn Bio, because otherwise only Luca can do any decent damage. On top of only having two party members (even if you repaired Calca and Brina you don't get them) and no White Mage, the normal enemies of the dungeon can be quite strong, and the final room has you climbing a series of ropes, pick the wrong one and you're thrown into a series of battles with no healing in-between. The dungeon's mercy is that the boss is easy to beat if you know the strategy, but the dungeon more than makes up for it. There's a save point to reset and heal after every floor in contrast to most other Challenge Dungeons that either have none or just a single one before the dungeon boss, thank the heavens because if you didn't get them it would be nearly impossible.
The Fork Tower is a handy example of how the precise opposite of a Marathon Level can still make a player hate their life. It's very small, thus has no save points within, for one. The gimmick of this tower is that it has two forks, and your party must split into two groups of two to traverse it. One half is physical and for physical fighters, the other half is for mages, and it's the mage half that provides all the agony. One of the random encounters on the mage side is the Flaremancer. It counters non-magical attacks with two hits of 9999 damage, a surefire instakill...and it considers the Geomancer's Gaia moves non-magical. It's the only thing in the game that does, too, and being that many players will give mages the Gaia ability to save on MP...you can see where this is heading. Then there's the Chrono Controller, who is much less tricky than the Flaremancer, and will simply cast Meteor and first-turn annihilate your two-person party if he so desires. But not even running from every random encounter will save you. If you make it to the top with the mage party, then make it to the top with the physical party, then beat the physical party's boss...there's a segment where you snag the magic at the top of each tower simultaneously. And they are serious about the simultaneous part. Have to go to the bathroom? TOO BAD. You get to do the whole thing over again. And after that, once you get back up the tower, there is a boss. A boss whose attacks require Reflect/Carbuncle up on all party members at all times. Because once Reflect wears off, he can one-shot any member of your two-person party as he pleases. He hit the healer? Or the person with Carbuncle? Or both? Sucks to be you.
To the unprepared, the Fanatics' Tower in can get annoying fast: long, no save points, very frequent Random Encounters, and only magic attacks allowed (by both sides) except if you berserk yourself. And at the end, the boss's dying attack is almost guaranteed to wipe out your party unless you knew about the attack beforehand and had learned and cast Life 3/Reraise or were willing to spend a long time draining his MP (or used the summon that makes your party jump...). If not, have fun traversing up the staircases all over again! (Possessing a hidden item that eliminates random encounters makes this area drastically faster.)
On the plus side, with some relatively easy preparation (read: equip relics that give your party members Reflect by default, and let the enemies' own stupidity do them in), it's also an excellent place to help your characters learn spells. Doesn't make it any less annoying overall, though.
Daryl's Tomb. With one exception, each monster in the area has the ability to set the then-nearly-unseen Zombie status on your characters. Said status is pretty much a combo of Dead and Confused and your game is over if all your characters get hit with it. Factor in the fact that one enemy can Zombify as many as three of your four active characters, and you've got a pretty quick death if you didn't come prepared.
Zozo is one of the first large challenges in the game. Despite being a town, it has random encounters, though the music and dreary appearance may be a warning. However, the enemies found there are incredibly dangerous for that point of the game, and you still don't have basic Esper taught magic. Outside the Hill Gigas will randomly cast Magnitude 8 when it dies, doing around 200 damage to the entire party, while inside buildings the Veil Dancer casts Fira, Blizzara or Thundara—against the entire party that's 200 damage, on a single target it's an instant kill. If you didn't bring Celes (read: the only party member with Cure by this point) with your party, you're pretty much screwed because you'll need constant healing and all you'll have for such is items. Later, Mt. Zozo has enemies with very high physical evasion, though a Sniper Eye to guarantee all attacks hit makes them more manageable.
The Phoenix Cave also has deadly enemies, but also features a very confusing double-party puzzle involving spiked floors, a two layered dungeon, and a pool of water that needs to be drained to open more paths.
The truescrappy of the Phoenix Cave, at least in its original SNES incarnation, was that this dungeon was where the critters with '1000Needles' showed up and corrupted your save.
If you don't have an Amulet or a Ribbon, the first part of the World of Ruin, before getting Sabin or Edgar (depending on if you're going for the Celes, Edgar, Setzer or minimum-steps chcallenge), is essentially a Luck-Based Mission. And more often than not, you're out of luck as they hit you with a zombie attack that almost always hits, ending your game.
Especially the collapsing house. Not only is there an unforgiving time limit, but it's full of chests with very good loot (including the Blood Sword and a Holy Rod). However, the place is infested with both Demonic Spiders and Chest Monsters, meaning you get to run through the house again and again, trying to memorize which chests to grab and which to skip, and hoping you don't get overwhelmed by Scorpions. Equipping Celes with Sprint Shoes and RunningShoes will give you a little breathing room, but it opens you up to the Hermit Crab's dying attack, Rock. You can skip it and come back later with a full party, if you don't mind taking a long trek up the Serpent Trench with only Celes in your party.
The Floating Continent. The enemies are more than a match for even overleveled characters. Ninjas will spam powerful elemental attacks that hit your entire party. Brainpans use 1000 Needles (Blow Fish in the SNES version) when alone and can inflict Stop. Behemoths have powerful counterattacks like Take Down (if it is attacked at all) and Meteo (if you summon an Esper that deals damage). Apokryphos have level-based, Status- and Instant Death-inflicting counters when they're alone. Dragons have Revenge Blast, which can do absolutely insane damage if you damaged it enough, and can possibly counterattack with Snort, which blows a party member out of battle. Misfits will shred you apart with Lifeshaver (although having Gaia Gear equipped makes it actually beneficial). Also, there's a brutalChest Monster halfway through, and it ends with a vicious fight with Atma Weapon, and a timed Collapsing Lair sequence with another boss at the end. This is the first big test for your character planning; if you haven't used the Espers intelligently, you'll get stuck.
The big problem with the Floating Continent is the arbitrary character limit; when you start the approach, the game only lets you take three characters and puts you through a series of fights, ending with two boss fights in quick succession (although having a Tent means you can heal up as soon as you land). You pick up Shadow right after landing, but if you didn't know this was coming and grind him on Crescent Island when you got the chance, he'll be underpowered, lacking any magic, and probably poorly equipped as well. If you give him an Esper with good growth rates right off, you can get some spells on him pretty quickly, but he'll still be weaker than anyone else in the party. Shadow gets replaced by Celes for the last segment, who is less underpowered, but still fairly weak. And don't forget to wait for Shadow!
After the Ultros fight, the characters get split apart and go on their own for three different scenarios. Locke's scenario is absolutely brutal; first, you've got to save Celes. Then you get to escort her through the dungeon. The problem is that she's totally without equipment - unless you had the foresight to complete Sabin's scenario first and buy her some gear, you'll be spending the first part of the dungeon running away because she'll go down in 3-4 hits. After you get her some armor, it's not much better - sure, she knows magic, but absolutely nothing down there is weak to Ice. Then you get to go to the Figaro Cave and face the Tunnel Armor. The saving grace of this scenario is that it's short.
Cyan's sidequest in the World of Ruin features an irritating puzzle, enemies that will almost certainly cast the Imp status on you, and ends with a long and annoying boss fight where you're required to kill off party members.
The entire Northern Continent can be infuriating depending on if you have a guide or not. We start with the long, maze-like and rather boring Lost Capital (Where a certain plot point takes place), moving on to a fun, but difficult-to-control snowboard segment that deposits our heroes into the Great Glacier, a gigantic area with annoying enemies (Those freakin' Hungrys), an unintuitive method for getting a summon, and a challenging segment where the screen rotates periodically, messing you up. If you manage to get through that, you'll have the pleasure of dealing with Gaea's Peak, an annoyingly huge mountain trek broken into the maze-like caverns (where you could have the misfortune of facing a Malboro), and the climbing segments where you have to take breaks to keep Cloud's body temperature up. All of this is capped with a horrendous boss fight against a two-headed dragon that comes out of nowhere just to wreak your stuff, and the area where you fight it is subject to a rare glitch that makes it so that you can't run away from any random battles following a battle that disables running away until you change screens or load a save, making it possible to be left drained of resources due to the boss battle and then get into a random battle with a near-dead party with no way out of it before you reach the save point. Luckily, after that is the Whirlwind Maze, where, while not a cake-walk what with the timing puzzles and a tough boss surrounded by long cutscenes, is still interesting with cool music and relatively few fights. It's also where a huge amount of the plot gets revealed and things get bad.
The battle between Balamb and Galbadia Gardens is not fun. At the start you have to divide your party, and then do it several more times forcing you to keep swapping your GFs around, your homebase is now filled with random encounters, then without warning you have to battle a flying enemy soldier in a rock-paper-scissors mini-game where your lifebar gets decreased by every wrong move you make even before the damned game even starts, the mini-game itself is frustrating as the fight is completely random and based on luck and a single wrong move can doom you and force you to start again. Then when you're finally done that you get to enter the enemy base, a maze filled with random encounters, some of which can be very dangerous depending on how high you've leveled, you have to locate hidden students to get the key cards to open the locked doors in order to get to the boss...or bosses actually since there's at least three boss fights inside, four if you want to get the Cerberus GF which is lost if you don't.
Lunatic Pandora is a nightmare. See, the monsters are tough, right? No big deal, it's late in the game, you'll be getting plenty of experience for each battle. But wait...what's wrong? Seems as though the monsters are only giving you 1 exp per battle. That's right - if you don't come prepared, you're screwed, and you can't level grind your way up to where you need to be.
The Desert Palace. Unnecessarily complicated light puzzles, completely full of extremely deadly random encounters, and finished with a potential That One Boss, who can be almost unkillable if you missed a few of the "bloodstones", completely optional item pickups that the game doesn't tell you about. Oh, and to make matters better, if you listened to Kuja's suggestions about party formation for the other dungeon, you probably sent all your heavy hitters there instead, leaving a bunch of undertrained characters and Squishy Wizards behind. The hardest monsters in the Oeilvert run are found in the short segment leading to the airship dock. Which, incidentally, is the Desert Palace.
And who thought Fossil Roo was a good idea? It separates two great portions of the game, causing the player to deal with puzzles where flipping switches will cause giant ants to take you to different places. Don't forget that if you miss an item and try to leave to reset the puzzle, it doesn't work. Random encounters occur every three steps, and the enemies all have annoying status effects; there are so many that you probably won't be able to block them all.
Insult is added to injury if you want to get the ultimate weapons, because you have to come back and dodge the lightning bolts here, which is already insanely difficult, but now you have to do it a bajillion times without missing a single strike. The only warning you have that it is about to hit is a flash across the screen a split portion of a split second before the strike hits. Not even timing helps, because that's random, too. You can try to take refuge under the lightning rods so that you can catch your breath, but the game usually rewards your pragmatism by striking the moment you come out of the protective range, which isn't clearly marked. There is a list somewhere of situations that you would actually prefer to go through while impersonating Alex from Clockwork Orange during his programming: eyelids forced apart with someone periodically applying eye drops. This is one of them.
How many is a bajillion? 200. Get hit after 198 succesful dodges? Tough luck, you have to start over from the beginning.
Another ultimate weapon requires you to win a chocobo race with a time of 0:00. This can be done since balloons on the track can be picked up which shave three seconds off your time. The catch? For some reason, tiny birds like to come out of nowhere and home in on you, and the impact will ADD three seconds to your time. They stun you as well, adding more to your time, and sometimes birds will strike you while you are already stunned. A wise man would simply throw his controller into the wall first and save a step.
There's also Home, which can be extremely challenging if you haven't been leveling up that much. In addition to the fact that there are Chimeras everywhere, they are often accompanied by Guado guards, and you can't escape certain fights. Good luck getting through this place without saving at least once. The constant alarms that sound like someone constantly yelling "I'm annoying, huh?" doesn't help matters any. Yes, Random Speaker Guy, you most definitely are.
The Thunder Plains New Cave that's unlocked after completing the quest in Act 5. A veritable maze with non-sequential doors that rely on adding up certain numbers that are given to you throughout. Have a pen and paper with you, because you will be referring back to it, as the combinations of later doors often involve the codes from previous doors.
The Pharos, an exceedingly long slog through Demonic Spiders and strange puzzles, one of which will cause you to be transported to a room with monsters that will kill you or wipe out half your party. King Raithwall, the last person to ascend the Pharos, left a message IN HIS OWN BLOOD by the entrance that pretty much said "Don't fuck around with the Pharos". Initially, it seems rater innocuous, but later on gives way to sheer obfuscation and ruthless enemy ambushes. A puzzle halfway up requires you to get rid of either your magic, physical attacks, items, or minimap, leaving you to wander aimlessly through blind corridors and dead ends as powerful enemies cut you to ribbons. The Third Ascent is essentially a twenty-minute boss fight, and discussing it would take several pages of text. All you need to know is that there are no save points, and the people at Square Enix are sadists (yes, real sadists) for what they put the hapless player through.
Throughout the Pharos, you have a Guest Star Party Member who you can't control and will attack the enemies you're supposed to leave alone. And they usually place the good and bad enemies right next to each other, so while you're killing the right enemy Reddas is off killing the wrong one. Your options are to equip your lead character with a long-range weapon and snipe at it from a distance or just kill Reddas until you're done with that section. (But then Reddas is a very welcome addition to your party because of the super-strong monsters.)
The Pharos is an instance of well-applied Refuge in Audacity. In a game that's full of long, difficult dungeons, Pharos makes for an excellent climax simply by being so breath-taking. The dungeon is so huge it could just about pass for a game in its own right; it feels like it's the sort of trial you'd really have to face in order to challenge the will of the gods. It becomes portentous and atmospheric and awesome by the token of its sheer size and the quantity of challenges stacked against you.
The Great Crystal, where the minimap is disabled, the main map is completely useless, and most of the enemies are Demonic Spiders, especially the cougar-type Oses, which use an attack that removes 75% of your current health and inflicts every single one of the game's myriad status debuffs. It's essentially a modern-day Magical Mystery Doors level, too, which is a huge pain when you accidentally start running in loops and don't notice because the area names are a code. In SANSKRIT.
Furthermore, the level design itself in the Great Crystal and Giruvegan is frankly very boring. The walls shift between crystals and mildew. It gets tedious and unpleasant on the eyes twice as quickly.
Those who dare venture past the Scorpio Gate into bonus-level territory face bombs that explode in packs, spell-spamming ghosts that sometimes spawn infinitely, zombies that enjoy inflicting multiple annoying status conditions and MP-stealing reapers. Often in groups of four or more. This gets to the point where finding a mini-boss enemy is actually a relief, because there's usually nothing else in that room. To progress toward one of the two optional bosses, you must make timed races through the area—through heavy enemy fire—to a force field. There is only one save point in the area, and accessing it pretty much requires a guide. Or a map. Or both.
The Necrohol of Nabudis. Optional, sure, but being swarmed by invisible Baknamies sucks. So much, in fact, that even Level 99 characters often die to ambushes. This is where the game's Infinity+1 Sword, the Zodiac Spear, is housed, and just running through the area to where the treasure chest is makes for more of a challenge than getting the other ultimate weapons.
The first area of the Vile Peaks when Lightning ditches the party, leaving you with Sazh, Vanille, and later Hope, none of whom have the Commando role yet. Several of the enemies you'll encounter in this area have high defense and HP, so the easiest way to defeat them is to stagger them, which is very difficult without a Commando. You could ostensibly avoid these enemies, but then you'd miss out on necessary CP. Granted, two members of your party have access to the Medic role, so you won't likely die. And all this is maybe an hour or so after unlocking the paradigm system, so new players are probably still learning the ins and outs and won't know the tricks that make this area a breeze.
The game's tutorials do explain that SAB skills will stabilize an enemy's stagger gauge... but you won't put that into practice for a while after you hear it. New players tend to struggle with this section because they'll have come to associate COM and RAV units with staggering.
Chapter 9 can be a big pain in the butt. Its very very long, the enemies are a noticeable step up in difficulty from the previous chapters, and there are a lot of them, and you have to play through a large section of the chapter with an extremely annoying alarm blaring in the background constantly.
The Fifth Ark, the location for the entirety of Chapter 10. Not only is it visually unappealing (it's a giant sewer/subway system, for God's sake), it's crawling with all sorts of Demonic Spiders and Goddamn Bats. On top of that, you basically do nothing but walk through tunnels, with the occasional enemy/treasure/switch popping up here and there. Lastly, there's barely any cutscenes; the most that happens is the battle with Cid Raines and Fang getting Bahamut, which are the only redeeming moments throughout the whole chapter. They said this place was for the Sanctum to test l'Cie? It probably just motivated them to shrivel up and go Cie'th. On the bright side, it's followed by Chapter 11, which is generally considered the best part of the game.
While we're on the subject of Chapter 11, let's not forget that portion can be a nightmare to newcomers. When you first enter the Archylte Steppe, even the weakest enemies can pose a threat, and stuff like a King Behemoth or Megistotheran can utterly annihilate you. Squenix was at least nice enough to Player Nudge you towards a chain of sidequests that will help you get strong enough to take pretty much anything, but it still takes a bit of time before you can handle most of the wildlife.
Academia 400 AF. As soon as you arrive, the Proto fal'Cie Adam freaks out and starts a Cie'th Apocalypse. Since Academia is full of tight corridors, overhead walkways, and hidden passages, often you can get sucked into a fight with enemies you can't see, evade, or get a Preemptive Strike on. They start out with Ghouls, which go down like chumps and drop Potent Orbs like they're going out of style, but soon they get replaced with the infinitely more irritating Taximnote spams Wound, tough enough to take several hits and Nelapsinote Dodges physical attacks, comes in groups of up to five. Have fun navigating a dark, mazelike city, in the rain, where your map is virtually useless and taking your time only causes more Cie'th to spawn. Even worse, a later Fetch Quest sends you back to find a Graviton Core (a small, almost invisible object) hidden in the city.
Academia 500 AF is an Unexpected Gameplay Change into a platformer. A platformer with dozens of rotating platforms and jumping controls that leave something to be desired. Even worse, the enemies in this dungeon are a major step up in difficulty from the ones in previous areas. If you didn't go and do some sidequesting and Level Grinding, then you'll struggle with even the weakest of enemies, and if you persevere, the Final Boss will stomp you.
Augusta Towers AF 200. It's a tower and you are locked onto floors, where you need to perform a series of puzzles of turning certain box-rooms into the correct position so you can proceed to your goal, allowing you access to the next floor. Aside from the music being practically non-existent in this area, the monotonous way of proceeding and an environment that gets visually boring very fast, the walkable paths are very narrow, with lots of twists and turns and it spawns medium and big sized enemies which often block your way. And due to the paths being so narrow, it's also difficult to try to avoid getting into a battle.
Temple Of The Goddess is one of the more annoying locations in the game: until you beat the boss, your maximum HP will slowly go down, there's a lot of jumping puzzles and a chance to fall down several levels and lose a lot of progress (although since you can save anywhere, this isn't a huge issue) and the later enemies include Ectopuddings, which are difficult to get a first attack on due to their ridiculously fast stretchy punches, need to be frozen with ice-elemental attacks before you can start killing them and which do a ton of damage with their Fira spells if your guard timing is off. The boss himself is also difficult to beat if you're not prepared and has an attack that will more than likely kill you instantly if you can't guard it properly.
Falls Basin is also the only area in the game which you can't return to. Which sucks for you if you didn't get the Heal spell and left (though it's kind of hard to miss, being right below the boss.)
The first visit to Merchant City Dorter from Final Fantasy Tactics. The preceding three story battles (and story battles are what take up most of the game; you usually have to be actively trying to find a random encounter) are all simplistic fights with low-grade soldiers and highwaymen. Then you find yourself in a fight with seven war-hardened veterans who have been fighting together as a unit for years and this is reflected in their behavior in battle. Even veteran players of the game usually have to reload a save and try this battle at least three times before attaining victory.
Conall Curach in the first Final Fantasy Crystal Chronicles. It's a Marathon Level set in an incredibly bleak swamp, in a game otherwise renowned for its Scenery Porn. The second part of the area has you navigating a maze section, where landmarks are incredibly hard to come by and the dreary atmosphere makes almost everything look exactly the same; you can be wandering around the place for half an hour only to discover you were backtracking without even realizing it. The place's best treasure is hidden behind a Guide Dang It of mammoth proportions, the solution to which is in an entirely different level. And when you finally get through The Maze, you get to a section utterly infested with Demonic Spiders, including one area where you have to fight fourBosses In Mook Clothing all at once, a fight harder than the actual boss of the place. And considering the actual boss of the place is That One Boss who constantly summons Demonic Spiders and can become intangible and is immune to all your physical attacks and whose attacks are very hard to avoid, that is saying something.
Pharos Sirius in Final Fantasy XIV is a rather tough dungeon even after it got nerfed. Mobs consist of zombie sailors that can hit like trucks, corrupted flan that will split up into smaller copies when its HP is low, and corrupted sprites that can use a highly damaging magic spell that can hit anyone caught in its range. The bosses are no slouches either; the first boss can summon zombie dogs in pairs and they will hit you with a debuff that makes you take heavy damage AND reduce your defense if said debuff stacks 3 times. The second boss is a massive bird who will go apeshit and cause near instant KO to the party if you destroy the boss' eggs too soon. However, when the eggs hatch, they release baby birds that will attack you and sometimes they will use a rapid fire attack that can hit you at least 10 times in a row. The final boss is Siren herself, who can inflict charm on players and if said players aren't cured of it in time, they get hit with another debuff that makes them attack the party. Siren also summons zombie sailors to aid her, including one type that will physically restrain a player so that they can't do anything until that zombie is dealt with. Siren also packs the Lunatic Voice attack, which damages everyone and inflicts Reduced Immunity so that healing magic is less effective.