Ouga - Gowcaiser
Sorry, but there is no other boss in any game that is as ridiculously evil as Ouga. And the fact you have to beat the wondertwins just to get to him... Get Gowcaiser, see what I mean.
On the whole, Square doesn't make truly evil bosses. I mean, they are pretty much shit easy. Just make sure you keep your health up and you'll win with a little endurance. Is that what a boss should be?
You know what - bosses outside the SNK realm are just outclassed. With the exception of SFA3 Vega, as he had that incredible Shadow Psycho Crusher, but once you know what you're up against, he's easy like Sunday morning. About as tricky as White in Real Bout Fatal Fury: Dominated Mind - although you have to give White props for cheating...
Here's EVIL...
the Black Pierrot in Magical Drop 2 & III. What a complete fuckster. I guess you have to play against him to know what I mean. The problem is getting to him, but the guy is completely sick sad hard. He's the Charles Manson of puzzle games too...
and for my number 3 - that old man in Metal Slug 1~4, who appears at the end of 4 as the boss.
Come on. I saved that asshole's life almost 7 years ago in Metal Slug 1 (he's the naked old man hostage in level 4 or 5), and this is how he repays me? And I have ot say that the final form is just wrong. I don't like bosses like that at all.
Anyhow, that is, without a doubt, the most evil threesome of bosses you will face in video games.