Soul Calibur and Tekken Dark Resurrection are both good. There's also the Samurai Shodown Anthology for those classics. Also can't forget the Capcom Classics Collections (Remixed and Reloaded) which - I think - both contain versions of Street Fighter.
Right now I am playing Naruto ultimate ninja hero 3. I like the battle mechanics since it's easy and I'm not good with fighting games in general like Tekken, Street Fighter to name a few. It's hard to pull of those moves, especially on the psp compared to a controller or joystick.