As long as people are recommending games, I'd like to plug Mortal Kombat for a moment.
Dropped to $30 on PSN so I bought it. Beat the ladder with Scorpion a little while ago. Just started story mode. So much fun. Yes the character models have dumbed down poly counts, but in every other respect the game is pretty great. If you have any nostalgia for the original 2D MK's back in the 90's, this game is also amazing fan service. Controls are perfect, frame rate is perfect (smooth 60 fps), and tons of content.
EDIT: And yes, Velocity and Mutant Blobs are great. Also, check out MiniSquadron. One of my favorite iOS games ported to PSP Mini where it gets proper controls. At its best on the Vita. I think it's still only 99cents.