Oh boy, where to start.....
Gnaw - Awesomenauts
This guy is just funny as hell. For one, his playstyle is annoying, which makes playing as him fun, because you'll occasionally you'll kill unsuspecting enemy players with weedlings and poison dripping through platforms. His voice is pretty funny, yet cool and adds a ton of personality to his character. Oh yeah, and he has a hilarious melee animation. NOM. NOM. NOM. NOM.
Ryo Sakazaki - Art of Fighting/King of Fighters
Originally I didn't think much of this guy, but after using him in KOF 98, and later maining him in KOF XIII, this dude kicks ass. His karate based fighting style is really cool, he does some badass looking attacks (especially in XIII!) and he rides his fucking motorcycle with his GI on. Seriously.
Shermie - King of Fighters
When I first played KOF, she was among one of the first characters I took a liking to. For one, I love her hairstyle, and her outfit speaks for itself. Second, she has some pretty awesome animations and throws, especially in 2002. Third, she's one of the few grapplers I've enjoyed playing as, so much that she earned a permanent spot on my main team in both KOF 98 and KOF 2002. If only she was in the newer KOF games. Oh well.
Ryuji Yamazaki - Fatal Fury/King of Fighters
This dude is fucking psycho. Oh, and he fights with one hand in his pocket too, I love that.
Terry Bogard - Fatal Fury/King of Fighters
I don't think he needs an explanation. Awesome character, awesome moveset, and he only became even more awesome in Garou.
also dat engrish
Tam Tam - Samurai Shodown
I'll admit, I didn't care for this guy in the first game, but when I played Samurai Shodown IV, oh man, I began to love this guy. He has a really unique looking stance because of how tall he is, his mask is really cool looking, and goddamn he has massive reach with that blade of his. Oh, and his grab when he just starts biting his opponent with his back turned. So freakish and awesome.
Sion Eltnam Atlasia - Melty Blood
Around the time I discovered Melty Blood, I instantly flocked to Sion because I liked her appearance, and of course, she ended up becoming my favorite character in the entire cast. Really fun character to use.
Squigly - Skullgirls
Ah, of course I had to post this one. When I first began playing the beta back in July when she was still unfinished, I wasn't really fond of her for some reason. However, skip ahead about a month and suddenly she became my favorite character in the entire roster. I love how she fights with leviathan, she has some really cool and hard hitting attacks (stand HP comes to mind) and I just love playing as her. I really liked her story and he art used in her story as well.