Must-have PSP games?

I am now certain that I want a PlayStation Vita. Since scrounging up the cash by February will be a task I figured I'd start my library with some, hopefully less expensive downloadable PSP games. I've never had a PSP so I didn't keep up with its games. So, if anyone has an opinion on what I should try out I'd appreciate it ^_^

Ok ,since I DO have a PSP, let me list you the best out of the system

1. Ratchet and Clank: Size Matters
2. Daxter
3. Secret Agent Clank
4. Gurumin: A Monstrous Adventure
5. Crush

First off, Ratchet and Clank on the PSP translates very well on the PSP and is considered a good choice when on the go, Daxter was a cool game that actually takes place before Jak II came to be, Gurumin was a excellent adventure that had wonderful music and voice acting (I need to buy it again since it was STOLEN in Las Vegas) and Crush is a great puzzle game that actually really requires to think outside of the box, but also use the mind as well.

Probably the best thing you can download for the PSP are the PSOne Classics from the PSN Store. I have Twisted Metal 2 downloaded onto mine, and let me tell you, nothing is more fun that driving around the world in an ice cream truck and just blowing away the competition. lol

Beyond PSOne Classics, there's also Twisted Metal: Head-On, Castlevania: Dracula X Chronicles, Parappa the Rappa, GTA Liberty City Stories, GTA Vice City Stories, Final Fantasy IV Complete Collection, Kingdom Hearts: Birth by Sleep. The last two I have yet to play, but I can say the others are some of the best games for the PSP.