What is the worlds hardest psp game

It entirely depends on your skills and how quickly you can adapt to a specific control scheme for a game. Personally though, I'd say that these games are the hardest:
- Monster Hunter Freedom (1 and 2):
This game is about... uhh... being a monster hunter, pretty much. The camera is rather irritating, but once you get used to the control scheme and work out a method of moving the camera quickly and easily, it gets easier. Doing hunting/slaying missions can be hard, depending on what monster it is, because the boss versions are so much smarter than the normal monsters. If any of your friends have a PSP, and the money to get this game (I got it for $20 AU), I suggest you both try out this game on multiplayer via Ad Hoc.
- Killzone Liberation:
Killzone Liberation is a tactical shooter game. The control scheme isn't that hard to memorize, and the game itself isn't actually that challenging either (unless you have trouble with shooting games). The game makes you go into a specific area of a map and see what the enemies do when you walk in. If you die, you do it a few more times and make a strategy. If you just run in and shoot everyone you can, you'll die. A lot.
- WipEout (Pure and Pulse):
Like Killzone Liberation, these games are rather easy. The control scheme is easy to use, and they're rather fun games. If you love hard games, these wouldn't fit you very well, as I only found them hard when you start them up. I don't play many racing games (especially not fast-paced ones) and during the first race, I couldn't manage to get out of last place at all. Its easy to win the races now though, unless you play against some talented friends.

I heard that Ultimate Ghosts and Goblins is also a hard game, but I haven't played it, so I wouldn't really know. Hope you like one of the games I suggested, and have fun!

- Angus