Jak & Daxter 1 (PS2) is the best one I've played so far. Difficult but not too difficult, large colorful worlds and plenty to do in each of them. Quite funny also.
Jak 2 & 3 were quite terrible IMO. Who in their right mind turns a cute platformer into a scifi shooter game :/ Basically a Ratchet & Clank look alike but done worse :/
Ratchet & Clank, also a good game series by the way. Quite a challenge.