Most difficult boss you've faced.

[jovack22]

Up there is "Shadow man" from Mega man 3.... no 'top man' power of course... I'm talking about fighting him with your basic "P" (later known as mega buster... or was it mega buster by mm3... the real question is who really cares... /rant)

 

[Headsprouter]

The Dark Genie at the end of...well...Dark Chronicle (or Dark Cloud 2 for you STUPID AMERICAN).

I love that game, but that boss is just a real pain. I never beat it.

 

[ShinyCharizard]

Most of the hardest bosses I have ever fought come straight from the Kingdom Hearts series. The hardest boss I ever fought and beaten? Young Xehanort on Critical in Kingdom Hearts 3D. Seriously, not only does he hit like a truck and have a ton of health, but once you get him down to 0 HP, he says, "Time Stop"! and a big clock is summoned, you have to kill the clock within a certain time period before you have to face him all over again, albeit with less health but damn it is still cumbersome. It's made hard by the fact that he constantly summons fast moving and hard hitting clones who's sole purpose is to keep you away from the clock.

Hardest boss I have ever faced and have never beaten? Mysterious Figure from Birth By Sleep. Oh I can beat Vanitas's Lingering Spirit but screw Mysterious Figure, that guy has a set of combos that can kill you in one go, extremely erratic and unpredictable movements, and he takes forever to beat.

This guy gets it. The bosses in Kingdom Hearts, especially the optional bosses, are incredibly difficult. More so then most any other game I've played. Games like Dark Souls ain't got shit on Kingdom Hearts when it comes to difficulty.

 

[MiskWisk]

ENOUGH! THIS...ENDS...NOW! [http://images.wikia.com/runescape/images/f/fc/Enough%21_This..._ends..._NOW%21.ogg]

Spoiler: Uncle Fester from Runescape.

Double triple quadra penta super duper ultra mega screw you.

Oh god, Him.
Dear god he was a prick, insta-death mines, cloning, insta-death attack, screw you attacks, op magic attacks, healing. This guy was the absolute bane of my existence. I ended up giving up and using the ruby bolt strategy if it could be called a strategy.

 

[Tyelcapilu]

The Necromancer in Castle Crashers is definitely up there as one of the most insanely tough bosses of all time.

 

[darkcalling]

The Hell Wyrm from Final Fantasy 12. Took me all freakin' night. Never did beat Yiazmat.

 

[PrimitiveJudge]

I am going to jump backwards about 25 years.

Remember playing the game and about 2 minutes into the game, remember the only weapon you have is a handful of rocks. ROCKS. Then this fucker shows up *boom* game over.

 

[Remaiki]

Well, Hibachi from DoDonPachi Resurrection (ah, the benefits of being in Europe) is practically impossible.

If we're only allowing bosses from games people (might) have actually heard of, I'd say... Byakuren Hijiri, from Undefined Fantastic Object (the 12th Touhou game). Holy fucking shit. Nirvana's Cloudy Way in Purple is bullshit, Magic Milky Way is bullshit, Devil's Recitation is bullshit, Superhuman "Byakuren Hijiri" is bullshit, Legendary Flying Saucer is bullshit. Her 3rd nonspell is also bullshit. (It'd probably be better to name everything that isn't bullshit...)
I'm tempted to name Yukari Yakumo from Perfect Cherry Blossom, but despite Mesh of Light and Darkness, Double Black Death Butterfly and Boundary of Life and Death, she hasn't got much going for her when it comes to Lunatic Byakuren Hijiri.