Not a fan of AI art in general, But who do you main in the Alpha/Zero Series (Akuma A-ism here)
Also consider 3rd Strike?
Alpha 2 is my favorite, i like Alpha 3 too but there is a problem in this game: V-ISM, i think V-ISM kind killed this game, not that fan of 3rd Strike honestly, it's a great game, but lacks charisma in comparision of the Alpha series, the soundtrack is worse than Alpha 2 too, there are tons of characters that i don't wanna see again in 3rd Strike like Twelve, Necro and Remy.
If i'll put a ranking, 3rd Strike would be my 4th favorite SF, would be behind Alpha 2, Ultra Street Fighter IV and Alpha 3(i like the story mode in Alpha 3 and the World Tour).
I didn't answer my main characters in each game, but, answering your question, my main on Alpha 2, i don't know, probably Akuma, i like to play with shoto characters in general, i like to play with Gen too(even he is a stance character), he is underrated in this game, in Alpha 3, my main it's Ryu A-ISM but i don't play Alpha 3 that much because as i said before, i hate V-ISM.
In 3rd Strike, Ken is my favorite, but i know that Yun and maybe Chun-li(at least in tier lists) are a bit better than Ken, i like to play with Akuma in this game too.