The title says it all.
Who do I pick? Well, here we go. The most annoying boss fight of all time with unpredictable attack patterns, annoying lines, the strange feeling of not being satisfied after killing him and a VERY difficult final boss overall;

JASPER. BATT. JR.

I hate this little twerp so much. He has everything I said above, (obviously) and MORE. Oh yeah, and unpredictable one hit kills.

He ruined a large amount of No More Heroes 2 for me, my life, my self esteem, my perception of reality, my sanity, and just about everything that lies in between.

I would rather drink greenish-red anal excretions from a cup made of pig crap.

Flame Hyenard.

BURNBURNBURNTOTHEGROUNDBURNBURNNBRUHRBBURehiwtwghwgoiewhgwrewt

Most challenging would be the fight against Ornstein and Smough.

Who to kill first? Kill Smough, and Ornstein gets hell of an enchant to everything.
Kill Ornstein, then Smough gets an enchant too.
Who to kill first? Try killing them at the same time? No, that doesn't work.
Good luck killing them on your first try without a guide. Except if you are a Demon's Souls player.

>INB4 phantoms.

1578449 Haven't we already done a thread like this?

Because I specifically remember talking about these two fuckers:

1578517 It's Drake and Josh!
For me, it would have to be...

1578449 Primal Dialga from Pokémon Mystery Dungeon: Explorers of Sky

This MOTHERBUCKER takes forever to beat, and everytime you lose you will lose about 80% of your inventory, with no way of getting the things back.
Seriously, most frustrating boss ever...

1578617 Oh dear mother of Christ, I hate Primal Dialga..

Annoying: Time Eater

This commie bastard right here:

He's the one Cobra I haven't figured out yet, and I've beaten Metal Gear Solid 3 three times and I'm currently in the process of playing the game on European Extreme. All the Cobras have a distinct pattern of brilliant strategy(Pain: use grenades. Fear: use thermal goggles. End: thermal goggles and sneak, counter-snipe, turn your console off and wait a damn week and he'll literally die of old age). Even The Boss is easier if you got the shotgun, but The Fury is the only one I can't figure out. I've equipped every last little thing I've found. Every weapon, every goggle, every little trinket, but nothing was definitively 'the one'. I have not found any perfect strategy or tool to use on him. Maybe he's supposed to make you rely on the one tool you start the game with; your brain. Or maybe he's supposed to throw off the player after they get used to figuring out previous boss's weakspots and trumping them easily. Whatever the case is, this guy has always given me the hardest time in MGS 3 and I'm not at all looking forward to how he's like on European Extreme.

sepheroth from the original kingdom hearts and to a lesser extent kingdom hearts 2

1578449 I think we've already done a thread like this... Anyway,
"DANCE, WATER, DANCE!"
That's all there really is to say on the matter.

1578858 Waaatttt? That guy is easy. I always thought that The Fear was the hard one.

The Pain? Meh.
The End? Legendary boss battle.
The Joy? Manly tears were shed.
The Fury? Easy.

Snappy the demon dolphin from Devil Summoner: Soul Hackers. Once you know what to do he's no big deal but what you have to do has no hints and goes against everything RPG players know. He absorbs magic so you HAVE to attack him with magic. Eventually he overloads and kills himself . I don't think anyone will blame me that I had to look up how to kill him.
The face of EVIL.

Night terror from soulcalibur. This guy will beat you to the ground as you try to get your bearings. And you cannot ring-out him.