Really depends what you're doing. If you're using official firmwares, use the latest one, but they're limited in the fact that they can only play umds and nothing like homebrew or isos. If you're using custom firmware, 3.90 m33 has the newest functions, but doesn't have as many themes and i believe Cintro won't work on it, so if you want to softmod your psp with Cintros, themes, and the like, 3.52 m33 or 3.71 m33 is the way to go.