Hardest bosses you ever fought

[Darko]

I'm only calling this a "boss fight" because Halo doesn't have boss fights, just incredibly hard sections of levels that essentially double as boss fights.

Truth & Reconciliation, when you board the alien ship. On Legendary, single player. So incredibly hard.

And I agree on the TMNT arcade game. Minus Stockman, though. That guy was a bitch. The first TMNT game for NES was almost impossibly hard after the 3rd level. I actually made it to Shredder one time. It took me all day to get there and the asshole kills me in less than a minute. :/