I'll probably go with Cyndaquil, for two reasons: I need one for the competitive team I'm building, and breeding the one from Colosseum with a Ditto to try to get one with the right gender and nature and high IVs is proving incredibly annoying; and Cyndaquil made for a very useful HMer in the old days. (Hrm, is Poliwag still going to be catchable early? Cyndaquil + Poliwag was a potent HMer teamup...)
...oh, and it just feels right for the rival to end up with a Feraligatr, just because that thing's so ugly.

As for the Kanto and Hoenn starters, I don't really care since you don't get them until the game's basically over. I don't have a passionate need to train any of them, so unless I want to breed something else and find a Kanto or Hoenn starter to be highly useful in a breeding chain, I'd be willing to get whatever the fewest other people get, to help with catching 'em all.