Hard to say.
Hardest Boss ever made? I'll give that to Noise Factory and Johann. Probably one of the cheesiest things to grace a Neo Geo. Perhaps the Cheesiest. In fact, he should name his moves after various types of cheese. That and the complete lack of recovery time needed for just about every move in his arsenal.
Meanest single Capcom Boss of all time? One particular game, one particular dude. Shin Akuma from Street Fighter EX (the first one). Tough as nails and also full of the cheese. He's up there, but not quite Johann. He's more of a Rugal 2002, 'cept with less (character-specific) weaknesses. Rugal 2002 would've matched the Akuma if he wasn't easy when you chose very specific characters. (Akuma mode SFA2Gold would be the hardest achievement, but 1 single S. Akuma (even with their insane speed and priority) from that mode doesn't match the 1 single S. Akuma from that game.
Most fair boss I remember in recent SNK history? Kain from Mark of the Wolves. Been a long time since I fought a boss in which I felt, if I lost, I lost fair and square. Hardly a cheese offering for that guy, if he beats you, he out-thought you. I personally like his A.I. a lot.
Gill, I do like. Definitely a step up from Bison. While I like Bison's character design better, I just can't fear Bison anymore. It's been years upon years since they made a Bison that's hard to beat. In fact, not since SF2 (Arcade version only) was he a challenge. After that, they tweaked him with so many simple weaknesses that even challenge characters can handle him quite effectively. Don't get me started on his Alpha 3 stuff. Heck, as long as you remember to jump only when he throws his psycho charge ball, and you remember to alpha counter, his X-version super is useless. Otherwise, he's a punch doll that loves taking a beating. Gill does have some cheese, but very predictable. He has location-based weaknesses (like Ignis) in which any character can put him away. (Can't count Chun Li, she has a cheese pattern against Gill that takes no effort).
Orochi has great design, but as an A.I. I like Rugal's difficulty better. Plus, Rugal also has some nice character design as well. Capcom's version of God Rugal was also pretty nasty as well.
Geese is the man, but I like him playable better than I like him as a boss.
Great to play as, a pain to play against.
