I'd have to go with Jak and Daxter. The Precursor Legacy is fantastic and while the sequels may not be as good they're still enjoyable. Even Jak X was pretty good (but nowhere near as good as Crash Team Racing).
From what I've played of the Ratchet and Clank series, the first game was really good and while I did play R&C2 and Tools of Destruction I never really got that far in either of them. I do hope to go back to them along with the others at some point.