Probably the Omega Pirate from Metroid Prime. I distinctly remember spending at least two weeks making runs at it, and when I finally took it down, I checked my pulse at about 150 bpm.
Omega Pirate? What are you talking about? He was easy. He's scripted and easy to predict, but even then he doesn't really do anything. Let's see, if you get too close he swipes you, if you get too far away he fires his cannon, but if you stay close-ish he just spams the shockwave repeatedly, which you can just jump over. When you break his armor, ignore the troopers and kill him as quickly as possible.
Meta ridley gave me a bit of trouble until I worked out that he opens his mouth for a second before he charges, and you can stop him with a charged power beam shot.
But if you thought that the omega pirate was hard, try beating him in a hard mode low percent run. Five missiles, no energy tanks and
no charge beam. In fact, parts of the game are much harder than any of the bosses in a low percent run. Remember the gauntlet in the phazon mines? There was a winding security room that was filled with pipey grating shield walls, and several sections where space pirates would pop out. There was a small section where you'd fight three our four wave troopers at once. Ucharged wave beam shots would take forever to kill them, and they'd all gang up on you.
The boost guardian from echoes was easy too. He'd boost in triangular paths, so you just had to stay inside the triangle. If you stood on top of one of the pillars he;d boost into it, break it, and you'd get a gold energy orb (the best one.)
The entire metroid prime trilogy was stupidly easy really. The only parts that I ever had trouble with were both in corruption, the third game, on the hardest difficulty. Phaaze in a low percent run was fucking hell. You had to avoid taking any damage because a single hit would most likely end you. A single energy tank just wasn't enough health. I could never avoid being grabbed by that stupid mouth thing in the pit. Gandrayda was awful too. She had so much health that it made the fight drag on for hours. She jumped around everywhere and was impossible to hit as well. Argh.
I just played Metroid Fusion again last week and this guy
WAS A TOTAL FUCKSTAIN.
I must have lost about a dozen times until I got his goddamn pattern down. I was also down a couple of Energy Tanks because this game is a shitty asshole about letting you explore sometimes and I forgot to look for them when I could have.
Ahh, Nightmare. He was awesome. If you charged your beam and held it, you could spin jump through his arms. Pressing the missile button while spinning would fire your beam, and if you kept the beam button held it would start charging again. Post faceplate flyaround is much easier with this method.
Boss Battles. Expert. Every boss from SSBB, at the highest level, practically all with OHKO attacks. Galleon. Rayquaza. Duon. FUCKING PETEY PIRHANA. And of course, tabuu. how we all hate you....
This. I still haven't beaten it on the hardest difficulty.
Also, I remember this part in Zelda: Twilight princess where you'd be out in hyrule field fighting this huge troll thing. You'd have to chase him at one point on horse.
He was riding a fucking pig thing. How the hell does a huge, fat troll riding a stocky pig outspeed a guy on a drafthorse??
from Bayonetta. He's one of the hardest bosses I've ever fought in any game ever. The difficulty is automatically set to nonstop infinite climax, which means no witch time unless you use the bracelet of time (you
don't want to do this. Waste of magic.) Or manage to get a counter with the moon of mahaa kalaa. He has eight healthbars, and a single flurry of punches knocks off 75% of your health in one go. Not to mention that charge > grab > sex move that empties your entire healthbar.
You have to spam the panther within to even have a chance of dodging his attacks. I don't mean running around, I mean pressing dodge constantly. It's insane how frequently bat within is activated during this fight.
I've only done it with the climax bracelet. Shuraba and fire durga are the best weapons for the job, using the PKP combo. The wicked heel on the second hit will stun him, and the final hit can strike twice at close range. Without the climax bracelet he just interrupts all of your combos, and your regular attacks do almost nothing. Although, if you manage to knock him off balance you can heel stomp repeatedly and make him bounce. Rofl.