In every installment of the Pokémon games to date, you will have different Random Encounters in different areas of the map. Pokémon that are numerous in one place are often unknown in another... except for the bats. Zubat, that is. Each and every cave is filled with them. Unlike the great outdoors, where you would usually be able to avoid random encounters by staying away from the tall grass, when you're in a cave everywhere is a danger zone and you find yourself traversing vast underground mazes laden with complex rock-smashing and rock-pushing puzzles while every third step you take, the screen flashes and you find yourself fighting yet another Goddamned (Zu)bat. These things are very fond of using a move called "Supersonic" to confuse your Pokémon into attacking itself half the time. Later in the game, they learn "Confuse Ray," which is like Supersonic but 100% accurate, and even gain the power to prevent you from running away, or even switching Pokémon, with the move "Mean Look." (They're hard to run away from even when they don't use Mean Look, because running is determined partially by speed, and the bats have pretty darn good speed stats.) To exacerbate things further, they happen to be unusually popular among whatever evil cult you're foiling. Popular enough to warrant carrying three or four of them. In generations three and four, they (logically, but frustratingly) even appear when you're swimming in water.
Taken from a report on Mount Moon in a FireRed Let's Play Thread by Red Chocobo:
Report: ...I'd like to make an aside and say this: Mount Moon has made me hate Zubat with no end. Zubat Zubat Zubat. Endless swarms of damnnable Zubat. I was breathing Zubat with every breath. I couldn't move my arm without hitting a dozen Zubat into about a hundred other Zubat, causing them to all get pissed off and start divebombing me... (it goes on and it continues about 3/4 down this page).
In later games, Zubat's evolved form, Golbat, is even worse: gets Poison Fang at high levels, which has a decent chance of inflicting high-strength poison on its target. If you didn't stock up on Antidotes and don't have the extremely rare "Aromatherapy" or "Heal Bell" moves, you'll either have to leave the cave to visit the Pokémon Center and start the dungeon over again or just watch as your monster slowly faints (with annoying screen flashes to indicate HP loss every few steps). Golbat also gets Air Slash and Bite, which have a 30% chance of making the target "flinch", essentially forcing it to skip a turn. Bear in mind that Zubat and Golbat do not learn too many moves while levelling up, so they can still be expected to have Confuse Ray and Mean Look at those levels. With all those Textbook Annoyer moves, it's almost like Zubat was designed to be such a fucking pest. The saving graces are that it isn't ridiculously fast (Base 90 Speed), will more than likely be a lower level than the Pokémon in your team, the flinch moves can only trigger the secondary effect if the user is faster, has poor defenses (75/70/70), and shitty typing.
In Unova, Crobat can now be found in the wild (post-National Dex). The encounter is rare, however. Also, Crobat's strengths as a Fragile Speedster are more than enough to make catching it a good move.
In Pokémon Black 2 and White 2, you can now explore the Castelia City Sewers, it has Rattata, Grimer and your old friends, the Zubat. Isn't that great? Though at least this time around they're only in that dungeon, instead of, you know, every single cave.
Though Golbat is in the Dreamyard, Celestial Tower, and Strange House.
Woobat are slightly more common in Pokémon Black 2 and White 2, but no where near as ubiquitous as Zubat. Most Woobat in the wild learn Attract, a normally Useless Useful Spell that causes enemies of the opposite gender to fall fall in love. Love-struck Mons have a 50% chance of being unable to act on a given round. Woobat have a 50/50 gender distribution, but as many species of common Pokémon have a skewed Male-to-Female ratio that could rival The Smurfs, a female Woobat is bad news. Fortunately, unlike Confusion, genderless pokemon are completely immune. Unfortunately, outside of events, most Genderless pokemon don't show up until after Woobats stop being a major concern.
Kalos only has Zubat in one small cave (called, appropriately enough, "Zubat Roost"), and even there they are rarer than a Pokémon fan may have come to expect — but the Woobats are all over the Coastal portion's caves. They like to swoop down at you if you cross easy-to-overlook shadows on the floor, and Repels have no effect on this.
There is also the equally frequent Geodude and its evolution, Graveler (often at the same caves, at that). While they tend to go down quicker, they have an inexplicable tendency to use the insanely powerful "suicide" moves "Selfdestruct" and "Explosion" before you manage to land the final blow on them. This usually screws you out of experience points and a usable Pokémon. Lovely.
They're quite fond of the move Defense Curl, which increases their already-high Defense stat and powers up Rollout (a STAB snowballing move). They're slow enough that you can generally easily run away from a fight with one in the wild though.
They're even more fun from Gen V on, where they have a 50/50 chance to have the Sturdy ability, which ensures they can't die in one hit. There's nothing so annoying as bringing a Graveler to 1 hp and then to see it blow up and take your own pokemon down with it.
For even more fun, Tentacool often know Wrap, which can prevent you from fleeing/switching, causes damage at the same time, and lasts 2-5 turns. This was even worse in the first generation of games, as Wrap prevented the target from doing anything.
Also in watery areas (starting from the third generation) are Wingull, who can learn Supersonic, like Zubat, but fortunately can't naturally learn Confuse Ray or other status moves. They are also fast. Fortunately, due to their typing, they are extremely weak to Electric attacks — meaning that strong Electric Pokémon can engage in Wingull massacres.
Wingull's evolution, Pelipper, isn't much better, as it's going to use Roost repeatedly and restore whatever HP your mon takes form it. You're in for a long fight if you can't OHKO it or at least knock it down into the red.
Magikarp, the most useless fish in the game. It doesn't help that they keep turning up whenever you use a fishing rod. Sure they can become awesome later on, but after you catch the one, you really have no need to see another one ever again.
Continuing the trend in the fourth generation, there is Bronzor. A Steel/Psychic type whose only weaknesses are Fire and Ground, and both of its abilities can cancel out one of the two weaknesses, meaning that each time you fight one, you have no idea which it's weak to. In addition to that, they also come with the move Hypnosis that can send a Pokémon to sleep, and have some of the greatest defense stats in the game. And there's only two Fire-type Pokémon (not counting their evolutions) in the game, pre-National Dex. Luckily, they're rather slow, so you can get away.
Fearow in the later areas of Pokémon Diamond and Pearl is another pain in the rear, because more often than not, you can't get away from them even when you hit the run button.
Diglett in FireRed and LeafGreen. Diglett's cave is full of them, they're fast, and there's a 50/50 chance of them having an ability that prevents you from switching or running way. Add in Sand Attack to lower accuracy and possibly Dig to avoid nearly all attacks for a turn and they get to be a pain.
Diglett and Dugtrio were even worse in Generation I. Not only did they have good physical sweeper stats (high speed and attack) and powerful attacks, but the game mechanics at the time based critical hit rate off of speed. Diglett's moves crit 18.6% of the time and Dugtrio crits 23.4% of the time with regular moves. If they have slash, a crit is almost guaranteed. If you didn't level up your bird, these guys become Demonic Spiders.
In Generation III and beyond, they are blessed with the abilities: Sand Veil (increasing evasion by 20% in a sandstorm) and Arena Trap (grounded enemies cannot leave). As if they were not annoying enough.
And then there's the inevitable power station levels, in which the Magnemite are customarily flying so thick it's amazing your character can actually walk through them. This would be merely annoying if you lead with a Ground type, which is immune to their Electric attacks... if they didn't have one move that does a flat 20 damage with no resistances (SonicBoom) and a confusion move (Supersonic). Did we mention that the Geodude family, the easiest Ground types to locate (seriously, they're everywhere), has a fairly low HP stat because it depends mainly on a good Defence and Resistance to normal damage?
Well at least they weren't given a move in Gen IV that makes them completely immune to ground attacks, because that would be stupid - Oh, hello, Magnet Rise.
Furthermore, their Steel/Electric typing makes them have the most resistances of any Pokémon, so good luck trying to inflict normal damage or higher on these without the correct moves. And in Gen V, even if you did, they may have Sturdy (see below), which means that they're not going down in one hit. Even more, if you've got a Steel-type out, you have a good chance of not being able to switch or escape.
And now the fifth generation has Roggenrola and Boldore, commonly found in caves. A Rock-type with good attack and defense, accuracy-reducing attacks, SELF-DESTRUCT, EXPLOSION, and the ability Sturdy. However, Sturdy has been buffed into giving the Pokémon a Last Chance Hit Point if it was hit with lethal damage when its health was full. Yes, that would mean that you would take at least two turns to kill one unless you had some form of multi-hitting move or the ability Mold Breaker, and if you're unlucky, it blows up in your face on that first turn.
Here's the clincher— in the Challenger's Cave, accessible after the Elite Four, you can find Boldore and Graveler together. Both have Sturdy, and Graveler still has its insane tendency to explode at the tiniest provocation. Enjoy your stay.
Thankfully Selfdestruct and Explosion have been nerfed and no longer halve your defense when it hits you. It still does massive damage, but at least now anything less than 20 levels above the user can survive it.
Gamefreak did attempt to balance this out by giving players numerous ways around the new Sturdy. Both Version Mascots, Reshiram and Zekrom, have glorified versions of Mold Breaker, as well as the extremely powerful Haxorus and its pre-evolutions, who have the actual Mold Breaker, allowing those three mons to bypass Sturdy. In addition, quite a few respectable mons know multi-hit moves that Boldore and Graveler are weak to, such as Cinccino's Bullet Seed, Vanilluxe's Icicle Spear, Sawk's Double Kick, and Emboar's Arm Thrust. While these moves aren't the best, they're good to have around if only to get around Sturdy.
And if you caught a Sandile or a Roggenrola of your own, giving it Rock Slide lets you deprive the Goddamned Rocks of an attack despite Sturdy 30% of the time. It's like Flinch's way of apologizing for Zubat and Golbat.
Of course, then they HAD to give Sturdy to a Sawk belonging to the Elite Four. Elite Four members have a Full Restore, so there's a good chance you'll be activating Sturdy TWICE (unless Throh is the one it gets used on, which is likely).
If that Patrat uses Detect one more goddamned time...
Its evolution, Watchog, can learn Hypnosis (puts your Pokémon to sleep), Confuse Ray (see Zubat examples), AND Super Fang (slashes your Pokémon's HP down to half). Have fun.
The worst part is Detect is supposed to only have a 50% chance of working if used successively. In practice wild and trainer Pokemon can use it with impunity and usually only stop due to A.I. Roulette, turning such bouts into a contest to see if you have enough PP on your moves to faint it before it decides to use something OTHER than Detect.
Looking for Pokémon in Unova's lakes, rivers, oceans, etc.? Hope you like Basculin! Trying to get rare/strong Pokémon from the special fishing/surfing spots in the same areas? Hope you like a slightly different Basculin.
Oh, and were you planning to simply run away from said Basculin, especially if your lead Pokémon has anything but top-shelf Speed? Hope you like being trapped while that Basculin pounds the crap out of you — the stupid little fish are fast and have surprisingly good Attack and pack a number of strong moves, like Aqua Tail and Double-Edge. And don't forget Final Gambit, if you happen to run into one above Level 51.
They're even worse in the Battle Subway. On the lower-difficulty lines, they will almost always show up on the 14th match and beyond, where they are very capable of ruining your win streak, pounding you with Aqua Tail after Aqua Tail and going first most of the time. Oh, and if you thought you could simply take them out in one hit, the majority of them carry a Focus Sash.
Durant. 40% Chance of seeing them in all floors of Victory Road, and you will be seeing a lot of them. The little jerks pack a punch with their 112 base Attack, and have a nice 109 base Speed to strike first with. They have some strong moves in Iron Head and Crunch when you find them, and they hurt. Woe to those who don't bring its one weakness, or catch a Heatmor along the way, because the little bugger has nine resistances, and an immunity to Poison. The only good thing about them? Base 48 Special Defense means any strong Special attacker will take it down in no time flat. Base 58 HP doesn't help either, on that...
Game Freak compensated for this one too. Half of the Durant get Hustle, which makes their attack significantly less accurate albeit significantly more powerful. And really usable Fire-types are found in abundance in Unova that are great for destroying Durants by the dozen. Black 2 and White 2 are even better, as Durant are only found in post-game areas.
Pretty much anything that knows Sand Attack or Double Team. Hope you have a move that can't miss or you'll be there for a while.
Or worse, both. One trainer in Relic Passage in B2W2 has a Grimer that knows both Mud-Slap and Minimize (the latter of which boosts evasion twice as fast as Double Team). Plus, it's difficult to exploit Poison's weaknesses that early in the game, and it has plenty of resistances for that point, as well as pretty high health for an unevolved mon. Which means that you'll have to get by with either low-powered Normal or weak types that it resists. If you even can hit it. Oh, the devilish thing also knows Disable as well, which limits your options even more.
Don't forget Trapinch either. While it only appears in four areas - Route 111 in Ruby/Sapphire/Emerald, Desert Resort,Reversal Mountain in B2W2-and the Lumiose Badlands in X and Y, it will appear every third step you take, and most will have the ability Arena Trap, which prevents your Pokemon from running away. Most likely they will also know Sand Attack, making killing them painful in and of itself. Even worse in Emerald, as you have to climb Mirage Tower, which is full of the things.
The Badlands in Gen VI is the worst for them, plus Gible and Dugtrio. Sure, Gible is a great catch but since repels do nothing against the Pokemon here and Dugtrio and Trapinch have Arena Trap...yeah.
Luckily, the little bugger evolves into Flygon, a very popular dragon type with great attack stats, making catching one a bit less of an annoyance, but after you do, its just like Magikarp; you have no reason to get another outside of a possible bad nature.
Staravia can be this if the Pokemon you're trying to level can't OHKO it. If it gets down into the red, it'll use Endeavor and bring you down to the same amount of HP as it. The only thing that keeps it out of Demonic Spider territory is its pathetic defenses to begin with, so thankfully this won't happen too often. A further issue is that Staravia get the ability Intimidate, which lowers your Physical Attack, so unless you have Clear Body or Hyper Cutter, you'll do less damage to them.
Emolga. It's lightning quick (no pun intended) and naturally learns Double Team, which raises evasiveness. Thankfully, they only appear in shaking grass.
Keep in mind we're talking about wild Emolga here. The Nimbasa Gym can be a major pain in the ass because many of the Trainers in there tend to use them. And that's not even mentioning the Gym Leader, Elesa.
Tranquill as of Black 2 and White 2 falls under this. Not only are they relatively common mid-game, but they are also annoyingly hard to fight due to their high speed. And, to make matters worse, they have a nasty habit of spamming moves like Roost (Which restores a good portion of their HP), Quick Attack (Which allows them to hit first every time), and Detect (Which causes your Pokemon to instantly miss whatever attack you selected that turn). This makes battling them (even with using Pokemon that have a type-advantage against it) an utter annoyance.
The entirety of Chargestone Cave in Black (2) and White (2). The encounter rate there is the definition of ridiculous. It won't be long before you'll hate seeing the Pokémon that swarm here (unless you run into Tynamo with its 2 or 8% appearance rate), and in BW the place is swarmed with Team Plasma grunts the first time you enter. B 2 W 2 doesn't, but it trades them for a few Ace Trainers with powerful Pokémon who fight you in Triple or Rotation Battles instead. To quote Marriland: "My best advice for going through the Chargestone Cave in any generation is to just use the stinking Repels!" Please, heed this advice.
Zebstrika from both Black/White and Black 2/White 2 certainly applies. For one thing, it has high Speed and Attack status, meaning it's likely to go first and hit hard. Second, it just loves to spam the move "Spark" in the wild, which has a chance of paralyzing your opponents. And, just to make matters worse, it also loves to spam "Flame Charge", a move that's not only super-effective against Grass Types (which normally have a resistance against Electric-Type Pokemon), but it also increases Zebstrika's already high Speed stats every time it uses it.
Want to make Zebstrika even more annoying? Try two of 'em. One pair of trainers in Black 2/White 2 use two Zebstrika against the player's Pokemon. This becomes especially obnoxious when Zebstrika's "Motor Drive" ability comes into play. Long story short, "Motor Drive" causes Zebstrika's already high Speed status to increase whenever its hit by an electrical move. Which move does the duo love to spam? Discharge, an Electric-Type move that hits both the player's Pokemon as well as one of the Zebstrika activiting the ability. Also, the move will paralyze your Pokemon (unless you're lucky enough to have a Ground-Type on your team).
Not a specific Pokemon, but the "Breeder" class of trainers in Black 2/White 2 certainly apply. They can be found on various different routes, and they want to battle every. Single. Time. You. Run. Into. Them. Yes, even if you've battled them before. Yes, even if you just entered a nearby building and exited it. On the one hand, this is very useful for level grinding. On the other hand, it gets 'very annoying when you just want to get from Point A to Point B without having to do battle.
If you want to get past them without battling, they're apparently nearsighted enough to let you past without battling... if you walk along the farthest wall/cliff/whatever in front of them. On a bridge? walk next to the water on the other side. So while they're not as bad as random wild encounters in caves, someone who hasn't figured this trick out will certainly find them just as annoying. The one that is not nearsighted like this is thankfully looking along the shore of a pond you'll only want to surf across to explore for an item.
Minccino and its evolution Cinccino (Though, thankfully, Cinccino is much rarer to encounter in the wild). While they can be defeated pretty easily, they can be rather annoying to battle. One reason is that one of the Abilities that they have increases the number of times a multi-hit attack strikes the opponent (So moves like Double-Slap and Tail-Slap are more likely to hit 4-5 times in a row instead of just 2). Not only that, but it can learn the move "Attract" (or some of them simply have the Ability "Cute Charm"), which causes your opponent to be Infatuated. What would otherwise be an easy battle is rendered annoyingly cheap thanks to your Pokemon being slapped silly while it's unable to fight back due to being in love with its opponent. Then they start learning Encore and Sing...
Any Pokemon with the ability Cute Charm (of which there are mercifully only 12...), which has a 30% chance to make a Pokemon that just hit it with a physical attack infatuated. Conveniently enough the Pokemon that can have it (with the exception of Milotic) also have skewed gender ratios in favor of females...
Oh, Liepard. Not only is it (like Watchog) found once the wild mons hit level 20, it's all over the place. They're very fast, and learn a lot of annoying moves like Fake Out (instant flinch, but only if used in first turn), Torment (keeps you from using the same move twice in a row, which can be bad if your options are limited), and Night Slash (increased Critical Hit chance on top of being a 70-base power STAB move). Until you've got a faster mon or some Bug or Fighting-type attacks, these cats will irritate you to no end.
Mienfoo are somewhat fast and difficult to run away from, and have quite high attack stats for being unevolved. Plus, they have very strong attacks like Jump Kick and Drain Punch, the latter of which heals them. And they probably know U-Turn, which is strong against any Psychic-types you'd want to use against them. To make them even more troublesome, they have such poor experience yields that fighting them in the wild isn't worth it.
The competitive metagame features one archetype known as the "annoyer", which is typically quite fast and is loaded with debilitating status effects. But because they're typically fragile, it doesn't take much to put them down for good. They can still cripple whole teams if they get the chance. For extra effect, they may have the Prankster ability, which gives priority to status moves. Notable offenders include Thundurus, Tornadus, Jumpluff, Whimsicott, and the recently added Steel/Fairy LIVING KEYCHAIN Klefki..
Trubbish in Pokemon Mystery Dungeon: Gates to Infinity, purely because they know Poison Gas. Poison Gas has high accuracy and poisons your target. This is bad because it also disables passive healing, and sapping 4 health every few steps. Unless you can find the stairs, you'll have to risk running into an enemy with lowered health, get by with only healing at low health, or use a precious Heal Seed to save yourself. Trubbish also have a good deal of health, and know Stockpile, which gives a boost to both of their defense stats.
Ghost-Type Pokemon in general fall under this in Pokemon Mystery Dungeon Explorers Of Time/Darkness/Sky. It's not their attacks that are annoying. No, it's rather the fact that they can travel anywhere. This means, most of the time, they'll pretty much spam their attacks on you (and your partner) while they're floating in a wall making them 'impossible to hit.
Shedinja deserves special mention. Unless you have an item to throw at it, or a team member with a super-effective move, it is notoriously annoying to defeat in battle.
Finneon and Gastrodon in the games are the bane of Water-Type players everywhere. To put it bluntly, these relatively common dungeon enemies have the ability "Water Absorb". Use a Water-Type attack, and they'll absorb it and increase their health. Did we mention this happens every time you use a Water-Type attack, even if you're not directly fighting them? That's right, a dungeon floor just needs 'one Finneon and/or Gastrodon to be in the area and it will screw you out of using your Water-Type moves. Now keep in mind that floor dungeons can have several of these Pokemon at once.
Nidoqueen. While not as common as other foes, she has the lovely habit of spamming the move "Earth Power". Not only does this cause a lot of damage to you (and your partner), she can also do it while she's out of your sight or in an area where you can't fight her back. By the time you even get close to her, she will have already made you waste tons of Reviver Seeds just from the sheer amount of times she uses Earth Power against you. And that's not even getting into times when there are multiple Nidoqueen.
DRIFBLIM. It's a Ghost type, so it can hide in walls, and both of its abilities(one of which enables it to attack multiple times if it doesn't have an item, and the other which causes an explosion once you beat it) are activated in these games...
Lampent, because they just love to hit you with Memento, which gives a huge offensive debuff to everyone in the room, and Flame Burst, which is a long-ranged attack that hits everyone around you for 10 damage. And unlike in the main game, they merely teleport away to use Memento again if they so wish after using it. They also know Night Shade, which deals a damage to everyone in the room equivalent to their level. They fortunately lack the hiding-in-walls ability of previous Mystery Dungeon games, but still...
Wild Floatzel in Diamond, Pearl, and Platinum. It's already quite fast and has moderately high Attack. But then it uses Agility, which raises Speed two levels, and Aqua Jet, where it always attacks first. It wouldn't be so bad if your chances of running away weren't partially determined by the Speed of the opponent...
In the original Ranger, Corphish were incredibly common in the Fall City Waterworks, which by itself would be okay. What brings it into this category is that it is an aggressive Pokemon, so it will run towards you every time it sees you, and there are often two very close together, so it's very likely that to proceed you will have to engage them.
There are also Arbok in the Sekra Range area, which are also aggressive and will always chase you.
Bidoof. It's one of the most common Pokemon you will most likely encounter in the early stages, Starly being another. Is it annoying to fight? No. Will it hurt a lot? No. Is it still bothersome? Yes.
Generation VI has some old and new faces for annoyance. Horde Encounters will take the troublesome encounters to new levels of annoyance, and the infamous Zubats, Geodudes and Graveler can appear in such formations, as can a few others. It is not only deceptively dangerous, especially after being repeatedly debuffed and chipped away at by a bunch of lower-leveled pokemon, but can be very time-consuming without area-of-effect attacks that can hit the whole formation.
A rarer encounter, but beyond irritating - Nosepass, apparently, hunt in packs. Between magnet pull and sturdy, it's going to take well over five moves for an underprepared steel pokemon to destroy the entire horde. That's five moves of, "The wild Nosepass used Harden." "The wild Nosepass used Harden." "The wild Nosepass used Block." Over, and over, and over.
Weepinbell and Spinda occur in respective hordes later in the game. Spinda will always spam Hypnosis or Teeter Dance to cause sleep and confusion, respectively, and Weepinbell uses Poison Powder to poison you.
Anything that knows Nuzzle. They tend to be very fast Electric Pokemon, making it hard to run from them, and Nuzzle always, always inflicts paralysis.
The Chatot along Kalos Route 12. In Generation IV, Chatot was a borderline Joke Character, whose sole reason to exist was Chatter, a gimmick move that interacted with the DS Microphone, and had increased power and a chance to confuse based upon the volume. Come Generation VI, and Chatter no longer interacts with the microphone; rather, it got buffed so it always confuses the target Pokemon. You can guess what the Chatot here love to spam, as well as Sing and Mirror Move. Even worse, they seem to somehow sense when you swap out a Pokemon, because they always open with either Chatter or Sing. They at least give decent experience upon defeat, but it's still infuriating to have half your party Chattered to death.
Route 12 also has Miltank, with great defenses and a really annoying movepool, and Tauros, with its annoying tendency to spam Rest, making it surprisingly hard to knock out. In fact, it's nearly impossible to level grind before facing Korrina, as all nearby locations have more than their fair share of goddamn bats/Demonic Spiders.
The drop down encounters in caves and Victory Road. Noibat and Ariados are tolerable, but sometimes Graveler show up. And outside on Victory Road, Skarmory,Fearow and even Hydreigon on rare occasions can dive-bomb you.
VOLBEAT AND ILLUMISE. While they're not that strong, they specialise in annoying the hell out of you by spamming Confuse Ray and Double Team. Just when you think you can hit them, you miss, and if your confusion expired last turn, you get a facefull of Confuse Ray again. And again. AND AGAIN. This is even worse if there's a Double Battle involving BOTH of them as the enemy, and if there's one, don't expect to get out of there shortly.
Talonflame. It has an ability, Gale Wings, that makes all its Flying-type moves go first. What this means is that you'll be on the receiving end of an extra-powerful Brave Bird that hits before you can even move. Talonflame is also extremely fast (so that even if you use a move that goes first, Talonflame is fast enough that it'll probably go first anyways) and hits quite hard. While it's fragile and is demolished by Rock-type attacks, it doesn't really matter because it's one of the most common Pokémon in competitive play.
Wash Rotom. This possessed washing machine makes trainers groan when they see it. It's Water/Electric-typed with Levitate, which makes it weak to only Grass-type moves (and Freeze-Dry), and has a good deal of resistances on top. It's got high defenses and can dish out quite the damage. It can cripple your team with Thunder Waves, Will-o-Wisps, and Tricks (with harmful items and Choice items). It's also one of the most-used Pokémon (depending on who you ask, the most-used) in competitive play, so trainers tire of seeing it constantly.