Street Fighter IV: incredibly pretty and refreshingly onliney, but a singleplayer mode tarnished by laughably long combo animations, a hateful boss fight, nasty AI and a seemingly total ignorance that some people haven't spent the last decade playing every bewildering, ultra-hardcore derivation of Street Fighter to death. Boo.
Street Fighter II: Biff! Kerpow! Thwack! Spinning biiird keeeck! That's the stuff. Pretend Street Fighter never stopped being The People's Fighting Game with the surprisingly robust (and official) Flash version below. The baying hordes should, however, be aware that this doesnae support controllers, so it's keyboard only.
Cursor keys to move, S,D,F for punches and X,C,V for kicks. There's also a slightly higher-res version here if you like.