The questionable honor of the most hated boss goes to one of the first bosses of the game, the Capra Demon. The boss itself wields twovery large machetes and loves to slam both of them down on any unfortunate player that thinks they have a chance of blocking its attacks. The two attack dogsthat accompany it make the fight all but impossible until they're disposed of, and the brutal speed at which the boss charges at you with powerful attacks as soon as the battle begins, combined with the very small area you have to maneuver in mean many players will stare at the "You Died" screen mere seconds after starting the fight.
Ornstein and Smough. They are essentially designed to be a co-op boss, and either one of them can kill a player in about two hits. Its nearly impossible for non-ranged players playing solo, as neither one will ever shift their attention, giving no windows to attack. To add insult to injury, they are also a sequential boss, in that if you kill one, the other gains his power and full health.
This gets even worse if you decide to kill Ornstein second for the Leo Ring. When Smough absorbs Ornstein, he just gains more health and stronger attacks. Ornstein, on the other hand, gains all of this and a completely different moveset, including the incredibly powerful Lightning Impale that provides the page picture and will almost certainly one-shot you if it hits.
The Four Kings. The battle is essentially a problem of finding a way to kill each King as quickly as possible before the others spawn while surviving/avoiding their highly damaging magic-based attacks. It gets worse in New Game+ since the Kings get a considerable health boost making it that much harder to kill them quickly. Oh, and just to make the fight even harder, the Four Kings do no physical damage at all. It's all magic damage in varying flavors.
It gets even better from there. What's your reward for defeating Manus, Father of the Abyss? You have to fight fucking two of them. Or, you could just run by them, but even that is difficult.
Artorias the Abysswalker is a likely contender for the hardest boss in the entire game. To wit: his attacks have a very wide arc and range, his pattern is too aggressive to leave many opportunities for healing, his attacks themselves do very heavy damage, he's extremely fast and his stage too small to allow for much manoeuvring, and when you just barely start doing damage to him, he'll go into a supercharged state where he'll get even more aggressive and dish out even more damage. Oh, and as an added bonus, he's resistant to most elemental weapons. Watch in horror as the Quelaag's Furysword that carried you though a good portion of the game deals little more than Scratch Damage to this demonic Lightning Bruiser.
The final boss of the additional content Manus, Father of the Abyss, attacks relentlessly and his arm can reach you anywhere... even behind him. While it is possible to summon Grey Wolf Sif to aid you, finding him at all is very difficult. The boss also has a huge amount of health coupled with insane resistance to elements, and his most of his attacks will likely break your guard in seconds. And that's not even getting started with the flurry of Abyss spells he can throw at you without a moment's notice, although they can easily be negated by a particular pendant, yet anotherGuide Dang It.
Gravelord Nito. He's not that powerful by himself, and most of his attacks are telegraphed. However, several smaller skeletons accompany him, and they respawn constantly until you defeat Nito. While you're busy trying to clear them off, Nito himself will start unleashing gravelord miracles at you or even try to take advantage of the situation by swinging his sword at you (even knocking off some of his minions in the process). Even worse, if you go too far into the boss room, you'll get swarmed by Giant Skeletons, adding to the smaller ones in case you haven't already killed them off with divine weapons.
Your journey to Lost Izalith is filled to the brim with bosses that will make you want to Rage Quit. To wit:
The Ceaseless Discharge. If you're not cheesing the boss around, then you'll find that he has nothing but powerful moves at his disposal, and his arm flail greatly suffers from Hitbox Dissonance, often nailing you when you think you dodged it. Straying too far from him will also prompt him to punish you with a fire blast that's unblockable by all but the mightiest fire-resistant shields.
The Demon Firesage. While it's mostly just a reskin of the Stray Demon, the room it's in is cluttered with numerous wooden branches and roots which litter the entire floor and can make your movement awkward. Some of these roots you can walk over, some of these you can't. Some of these roots you can break, some of these you can't. No indication as to which are which. This all being in a boss fight that's heavily reliant on your ability to dodge and get around the boss who can kill you very quickly if you're stuck in place.
The Centipede Demon. You're fighting it in a room full of lava, with a small strip of rock near the entrance. The Centipede Demon has several alarmingly powerful attacks with excellent reach, meaning it can hit you from a good distance while you're helpless to do anything about it. Worse, you get the Orange Charred Ring, which you need to walk on lava, after you defeat the Centipede Demon. You can get it early, however, if you don't mind trying to cut the Centipede Demon's tail... which is often over the lava lake.
The Bed of Chaos. If there was one endgame boss that is absolutely hated by players just for how mind-boggingly frustrating the strategy to defeat it is, it's this monster. Trying to get the full access to its core requires you to destroy two weak spots that become increasingly difficult to reach with the chaos that piles up throughout the boss fight (read: tree arms swinging around wildly, crumbling floor, fire scythes digging their way to you, and straight-up Firestorm pyromancies).
Standard result of a fight against the Smelter Demon.
The Ruin Sentinels. Their fight is similar to Ornstein and Smough, but it's a trio. It doesn't help that the battle starts with you falling (and taking a small amount of damage in the process) and a Ruined Sentinel spawns right next to you, ready to attack.
The Royal Rat Authority is a Flunky Boss the size and speed of Great Grey Wolf Sif, whose mooks can inflict toxic on you. He also suffers from Hitbox Dissonance, so some attacks will have problems connecting. Yours, that is. Not his.
Despite being an Anti-Climax Boss, the Old Iron King strikes fear into the heart of anyone who doesn't spam lightning miracles at him and are forced to fight him head-on. Punches that can drain your entire stamina bar if you block and flatten you to the ground? Check. Fire breath that also screws with your stamina management? Check. A conspicuously small boss arena which he can abuse to knock into a random crack of lava pool? Check! Its to the point that many posters on message boards claim that they've died to the Old Iron King's boss arena more than they have to the Old Iron King himself!
However, NG+ (or a patch rework) added a new ability to punish those that liked to snipe from the corner - a red-hot beam of magma that pierces through the wall and can knock the player backward into the nearby lava.
The Pursuer's Hit Box Dissonance with his attacks, especially his ability to curse you, has him take on this title as well. And on New Game+you get a chance to fight two at once!
Remember when the Capra Demon used to slaughter you within seconds of entering the fog gate due to the annoying attack dogs that made a perfect job at distracting you? The Flexile Sentry in New Game+ gains two nasty minions that will keep you on the edge with toxic buildup. And this is at a point where you should be ready for anything after the Final Boss.
The Lost Sinner has quickly become one of the most hated bosses in Dark Souls II. Not only does she have swift yet powerful, long reaching attacks, her arena is fought in the dark, meaning your lock-on range is terribly short unless you defeat another challenging optional boss, allowing you to light up the lamps in her room. What cements her status however, is when you lower her HP to 60% in NG+,two black phantom pyromancers spawn, both whom have access to some of the most powerful pyromancy spells in game. It's gotten to the point that both of their health and damage output were reduced in Patch 1.03 calibrations 1.04 due to how frustrating this boss was to many.
Velstadt, the Royal Aegis. While his attacks are easily telegraphed, what makes him dangerous is his serious case of Hit Box Dissonance; you'll be getting clipped for massive damage even if you rolled out of the way. He also has a crazy habit of mixing up his attacks; sometimes he'll string his attacks together, sometimes he won't, something which can throw your roll timing off if you think he's going into a attack animation. Blocking won't be much help either, since he hits like a freight train to your stamina. And once you damage him down to 60% percent. He'll begin to channel Dark Magic and start using Hexes. Which alone is bad, but it also gives him a massive boost in strength while along getting his defense buffed to the point your strikes are just Scratch Damage.
The first phase of Executioner's Chariot can be extremely frustrating for some. Playing solo, the respawning skeletons don't hit very hard, but they just love to pin you up against a wall and stunlock you, making it very difficult to escape and proceed down the corridor with the extremely small safe zones the arena gives you. Playing online, the chariot's trample seems to gain a very frustrating case of Hitbox Dissonance. Occasionally you will be at the back of one of the cubbyholes, with your partner slightly further forward, and you will get trampled while your partner is fine.
The Belfry Gargoyles in Belfry Luna. They wouldn't be so bad if you didn't have to fight six of them, possibly at the same time if you aren't careful.
The Ancient Dragon has gained notoriety for the most tedious and aggravating endgame Bonus Boss in the Souls series. For starters, all of its attacks are One Hit Kills unless you stacked maximum fire resistance (which can only help you for so long), has impossibly large area-of-effect attacks that force you to back far away from it, and possesses an incredibly random attack pattern depending on the direction and distance your're facing it from. There's a reason fans look down on this dragon as a Memetic Troll of the highest order.
The Throne Watcher and Defender have received a lot of flak for how they embody the greatest thing that could go wrong with a Dual Boss fight: both of them have unpredictable attack pattern, forcing you to wait until both of them are left open at the same time.
The Gank Squad (aka Graverobber, Varg & Cerah) in the Cave of the Dead are extremely difficult to beat solo—borderline impossible with certain builds. There are 3 of them, one that is like a tank (as in he's a Havel clone), one can make you bleed in about 3 hits and the other spams you with highly accurate and moderately damaged Dragonslayer Arrows. However the biggest issue is simply their large health pools, impossibly high poise, and how they keep together constantly, which make it almost impossible to single out any one of them on your own. Oh and they HEAL.
The Fume Knight has a large amount of things working to make him possibly the hardest boss in the game so far: He keeps switching between attacks with the short sword (fast and numerous, easy to block and dodge) and ultra great sword (slow but with reach that's even longer than it looks from the angle you view while locked on), all of which he can recover from somewhat faster than most bosses. The slightly uneven terrain can result in his swipes passing over you while you're standing or hitting even as you try to roll under. If you try to use Estus, he will stop whatever he was doing to interrupted you if he thinks he can, and he's almost always right. He has minimally-telegraphed, wide-arced sweeps to his sides specifically for people who try to continuously circle-strafe him. Then he Turns Red, completely turning the fight on its head: blocking goes from your best defense to almost worthless because his attacks all do three different types of damage, which almost no shield can handle. But they're also extremely hard to dodge because they all involve said ultra greatsword (moving much faster this time) and/or a huge area of effect. And to make it worse, the conditions of what triggers it are so poorly understood that it becomes effectively luck based: sometimes it happens when he's got less than a quarter of his health, sometimes he'll do it with more the three quarters of it left. If you wear part of a certain armor set, he'll do it immediately. If you don't have enough Smelter Wedges to deal with the Idols surrounding the boss arena, you'll also have to keep him away from them, otherwise he'll constantly regenerate health thanks to their wide-range Warmth pyromancy. This is likely to happen unless you had the foresight to save four Wedges, since one Smelter Wedge is only accessible after defeating the Fume Knight.
Just when you thought you couldn't have had enough of the Smelter Demon, the end of the Iron Passage introduces you to a blue version which is not only more powerful and has a few new moves (as befitting of a brutal Downloadable Content boss), but increases its Flaming Sword's range, making for more hitbox shenanigans. Oh, and every time it plants its sword, expect it to explode in your face. And since it does magic damage instead of fire damage, no shield can protect you completely.