Right now? Its Jpcsp. But its a hassle to use, most of the time, with lots of settings that take a while to get right for certain games.
Coming up extremely fast is PPSSPP. It literally went from inception to some games playable in a few months. Every day, the GIT changelogs are massive. Should be a few more months and i would guess it would play most games pretty well. Currently though, i would hold off on it, since its missing quite a few features.