Vega definitely. I think he's still got one of the best character designs, good costume design, good personality design, he's unique you know. He's regal, majestic, refined, graceful, beautiful, handsome, strong, silent, deadly arrogant and all the other qualities he has round out his character in a satisfactory way.
He's always been my favorite and I've always played best as him. I'm almost never beaten and his moves compliment my style. His speed is amazing, and his claw gives him extra reach. He can also pull a swan dive out of nowhere, and he's got several tricks to get himself out of a bind.
The only thing I didn't/don't like about the character is the change of direction they gave him. He used to be cooler before, but in recent games he's just portrayed and degenerated into this perverted, mentally disturbed sicko whose into pain, and thats a big change from the original version of that proud, silent madador who can dispatch people like an assassin (though he still can). Alot of people assume he wears the mask because he's nuts or something, but real fans will know he wears the mask because he wants to keep his pretty face from being damaged, he's that vain and so the mask has become symbolic. Its to his advantage in combat though, as it sort of freaks his opponents out, that and his claw.
Even with these radical changes, he's still my favorite.
Quick factoid: Vega originally had brown hair. It was changed to blonde later on because his strong spaniard roots are implied in his character and costume, and as we all know, the spanish have very fair, light skin and often have fine blonde hair; which leads to them, in the States and elsewhere, often being mistaken for caucasians.
Anyways 2nd in line is Chun-Li, I'm really good with her and I like her moves...and..well, her! LOL ^^