Loved this series since I was little, exploring the hugely varied planets with an arsenal of wacky weapons was just brilliant! If I had to pick a favourite, it'd probably be between R&C (original/2002), R&C 2, R&C 3 or A Crack in Time - I'd probably have to play them all again to decide as it's been a while for most of them.
Hard to say on favourite planet, but a few do stick in my mind, for various reasons:
R&C 1:
Orxon and Pokitaru - this was such a fantastic contrast, with creepy, depressing Orxon being followed by bright, sunny Pokitaru before going back. It was also a clever level design in that you needed to get items on successive planets to complete objectives on previous ones around this part of the game.
R&C 2:
Joba - had a really chill atmosphere/ soundtrack and I still remember all the attempts at completing the 'impossible challenge' in the arena xD
Silver city/ Boldan - that theme tune tho
R&C 3:
Thran asteroid - simply nightmare fuel lol, those qwark bots
Courtney gears boss battle... a big plasma whip and a gun that squirted white gooey stuff on her is what really sticks in the mind about that encounter
It's a shame the later releases of the game have gotten more and more 'meh'. Apart from A4O, ToD and aCiT, all the PS3 games were really truncated and overly dumbed down compared to previous games. The rebooted original for the PS4 wasn't terrible, but it deviated from the plot of the original (apparently to follow the plot of a film that tanked at the box office anyway) way too much and cut masses of what made the original game so special out (for example like I was talking about above with the level backtracking, but also more core things, like the developing relationship between the two titular characters, and for absolutely no reason I can see brushed Drek aside as the main villain and made him into a derpy stooge).