Unfortunately, there is no official PSP version of God of War Ragnarok available for download. The game was not developed or released for the PSP, and it's not possible to download a legitimate copy of the game on this platform.

If you want the "best" PSP file download for God of War, ignore Ragnarök and search for "God of War Ghost of Sparta ISO" (legally). It is the peak of the PSP’s capabilities.