Haven't played Generation IV or V yet but if I did, it would probably be Piplup and Oshawatt since I always pick the water type. Although I'm thinking on playing Platinum soon, and Piplup doesn't interest me as much as the other two. Hell, even in X/Y I'm more interested in Fennekin than Froakie.
Well, classically I've always liked water over all. Squirtle, Totodile, Mudkipz, Piplup, Oshawott. Then second choice kinda goes random. Charmander, Cyndaquil, Torchic, Turtwig, Snivy.
But this time... it all depends on third evolutions. Before I could always base it on the first. MAYbe the second (Oshawott, I'm looking at you). I'm not a fan of Froakie. Not at all. If it has some badass (second or) third form, WAY cooler than Croagunk (but along the same lines)... then I could consider it.
So for possibly the first time ever, I prefer the first-form Fire starter to first-form Water starter. It's eating away at me.
I gotta say though, as far as third forms are concerned, I may not have always chosen my favorite.
I mean, let's be honest, in Gen 1 (graphics), Blastoise was a mess. Feraligatr was great, save for looking kinda fat... again. In fact, fatness seems to be a big trend in water types. They're cute, then they're badass (well, okay in 1 and 5 at least... the others were all fat!!), then they're weird or fat or something. But I love them anyway.
"The point of war is not to die for your country. It's to make the enemy die for his."