Yeah, see I'm a die-hard R & C girl. They were the games that made me want to get into making games (and subsequently get my degree in making them). I love the stories and the weaponry... oh I do LIKE the weaponry!
Personal fave has to go to Deadlocked (or Gladiator as it was released over here). I really liked the arena combat and the Supernova was pretty awesome. As for variations on the RYNO, I like the Rynocerator - I quite like the fact that I can turn everything into dust with a reasonably rapid firing rate.
I think this game was pretty good, given that it's an internal storyline. I suppose that you have to think of it that way, rather than a franchise-long story arc. After all, the first five games were stand-alones and were quite brilliant by just playing off certain things that happened, rather than building on them... I don't think I'm explaining it rather well, but I know what I mean - each game acknowledged the previous ones but didn't continue the previous one's plot.
Either way, I still love those games and would continue buying them if any more were made.