Heh, Ruby Weapon... now there's a fight that i had to retry several times, and eventually succeeded after 2 hours (for a single battle). Definitely one of the hardest (and longest) battles i recall. Although fighting against Sepheroth after you hit level 99 is way too easy...

There was also the Epic Battle flash games, which are constant grinds by themselves. Hmmm

What other difficult bosses were there. Compared to Ruby Weapon very few bosses come close. Well there's the extra bosses in FF-X and FF-X2 which are meant to be impossible. I don't think i ever beat the second disc of .Hack because of the final boss in there. The neutral Ending of UnderTale was quite annoyingly difficult (and creepy looking).

Then there's non-bosses, like the levels above floor 60 in FF-XII which is just a pain to fight through until you're much higher level.

Hmmm... Everything in Bayonetta on Continuous Climax difficulty is annoying...

I like roguelikes, but they're pretty much a series of "defeated by" entries and an occasional "somehow didn't die until the next turn happened" so some defeats are less heroic than others.

Defeated by: Atamathon and Linaniil (Nightmare & above); a sickly diggle; the tutorial level; Jon Goddamn Irenicus; Muton Berserkers; that bandit at the target practice board south of town in QfG1; a vorpal bunny; the first party of bow-wielding kobolds outside Candlekeep; every opponent in Crusader Kings 2 ever; procrastination.

list TBC

Defeated by... procrastination? Yeah, that is one sneaky fuck, if there ever was one.

My toughest boss is always online DRM that kick me out of the game when the connection chokes from time to time.

Other boss I can use cheat, mods, hack that no boss is unbeatable. But this boss, no matter how much I cheats, I cannot prevent it from booting me out of the game.

Metal Gear Solid 2: Sons of Liberty - Solidus Snake

I had to become like a zen master cyborg ninja, practice, super fast reflexes and knowledge of his attacks. He was a real bastard. But not the hardest I fought.

Props on the Combichrist mention!

Toughest for me is probably the minion with a gun in Bushido Blade. In Bushido Blade you can die in one sword slash, much less one bullet. You have to hold off your attack until your opponent is ready or you'll lose honor and will get a bad ending, in this case it'll automatically end the playthrough. So you have to wait for him to be ready, hope he misses and then give 'em a good slash of your sword. If successful the boss fight takes all of thirty seconds (including his intro) but it's really tense.

Duriel

The final boss in Freddi Fish 2 is one of the hardest bosses ever. I spent months honing my skills until finally I could
S rank that fight no problem. B)

I don't remember his name but he was the final boss of New Legends, which is a little known game on the xbox only I believe. It was brutally difficult and took me days to beat. Me and my older brother started calling him the big cheese as it seemed literally impossible. The final boss in Lost Planet Isenberg gave me some trouble as well. Other than those ive not had any real trouble fighting a boss in a good long time. I'm sure there have been some others but none that really stand out as unbeatable.

Of course, we wern't born yesterday around here. :)

Digital Devil Saga - Fly Beelzebub

Dekaja, Dekaja, Dekaja. I won't forget the name of this spell ever after him. He will boost himself up indefinitely with no limit until he destroys your whole party easily. Stop him and he's meat for your demon grinder. F**kin Duriel deserves a special place in this thread.

Four Kings on Dark Souls on New Game ++

That was hard >_<

Recently, I've hit a ceiling with the final boss of Chantelise, second form. I might grind / fish for better equipment but it just feels so tedious. Leilodin, and to a much lesser extent Eye of Myrgato, from Valdis Story: Abyssal City made me rage for sure but at least I overcame those.

Nurburgring in Grand Prix Legends.

Every time i got to race in that hell of a track, it felt like i'm against the toughest boss. Driving slowly & safely around the track to learn it was one thing... but going through 160 turns without making a single mistake while at the same time waiting patiently for the driver in front of you make *that* mistake so you can strike or trying to maintain the best racing line (without getting too anxious) so as the driver behind you won't be able to pass, was the hardest thing in all my gaming experience.

Conker: Live and Reloaded - Alien

*Spoiler*

I felt a great sense of accomplishment throwing this f**ker out the airlock, three times...

You have to time those throws, and defend with the Loader suit when he tries to claw at you.