1. Monster Hunter Freedom Unite (or Portable 3rd)
2. Crisis Core: Final Fantasy VII
3. Ridge Racers
4. God of War: Ghosts of Sparta
5. Lumines II
I don't care much for GTA but either Vice City Stories or Liberty City Stories probably deserves a spot more than something like Lumines if I were trying to craft a "5 must-have starter games" for the system. Tried to make sure the list is varied in terms of genre. Monster Hunter may have some overlap with God of War as 3D action games but they obviously are super different in gameplay, structure and appeal. It is always tough to narrow a system down to just 5 essentials because the PSP has so many amazing games in a variety of genres so no matter what 5 you pick there will be blindspots.