I recommend "Burnout: Legends". Very good racing style game with serveral game modes that make it enjoyable. It has great replay value as well. In order to run it on a 1.5 though, you either need to use WAB version changer or No Update Launcher.
Also, try searching this forum using the search button. There are several threads with people debating the best/favorite game for the psp. Also check the Game Reviews section