For me its gotta be RAC3, just an all round great game. The whole series was great though, except for deadlocked - wasnt such a fan of that
my order:
RAC3: Up Your Arsenal
RAC1
RAC: Future Tools of Destruction
RAC2: Going Commando
R: Deadlocked

Never got to play any of the psp games, not to sure what theyre like...

I'm gonna say Going Commando or Up Your Arsenal.

Deadlocked wasn't that bad.

For me I would have to say the original...I love the game and the rest of the games just felt like more of the same without much change. They may have been done better, but it wasn't the same as experiencing everything fresh with the first game.

Hmm... I've only played the first 3, so I'll rate them from favorite to least.
Ratchet and Clank: Going Commando
Ratchet and Clank
Ratchet and Clank: Up Your Arsenal

Up Your Arsenal was my favorite, closely followed by Tools of Destruction. ToD's only problem was that it was a total cakewalk until the last level or two, otherwise it'd probably be my favorite in the series. After those two comes Going Commando, then the original, and finally Deadlocked. I actually thought Deadlocked was a pretty good game. It just wasn't a good Ratchet & Clank game, if that makes any sense.

" I actually thought Deadlocked was a pretty good game. It just wasn't a good Ratchet & Clank game, if that makes any sense."

I really can't decide, all 3 of those games are just so much fun and replayable in their own respects:P.