For me, Ratchet is the best, although I admit Deadlocked was the weakest one (still a great game IMO), but the first three Ratchet & Clanks are too sweet.
In a very close 2nd, I'd say the Jak series. I absolutely loved Jak & Daxter, but was initially disappointed in Jak II. Since I got into Jak 3, it got me back into Jak II, where I like it now. I also really, really like Jak X: Combat Racing, despite what some say. Daxter is awesome too.
The Sly series is my least favorite (I even like Ape Escape better). Although they have nice graphics, I just don't find the series to be as enjoyable and as addicting as the R&C and J&D games.
My order would be:
R&C: UYA
R&C: GC
Jak 3
Daxter (although on PSP)
R&C
Jak X Combat Racing (although a racer)
Jak & Daxter
Jak II
Ratchet Deadlocked
Sly Cooper
Sly 3
Sly 2
[quote name='Fatesealer']Well, for R and C, it sort of makes sense to add the weaponry... Deadlocked is the zenith of this theory. Although the fifth game damn well better go back to the roots...[/QUOTE]The gme wasn't really intended to have a fourth one, but since Sony wasn't releasing a new console (where PS3 was initially expected to be 2005), so Insomniac kind of had to come up with something. Deadlocked was mostly designed to be more action and online oriented.
Well, the next Ratchet, Size Matters, for PSP, will go back to the roots. Ratchet & Clank Next for PS3 should be the same way too.