In my Heartgold I chose Totodile because he's awesome and water types help a lot :3
Exactly this. Infernape really is a brilliant pokemon for the game, particularly as there is a lack of any decent alternatives until later in the game. The same logic goes into my choice in HG/SS, I pick Totodile because Feraligatr is useful for teaching Waterfall to, and there are decent fire types available in that game. Of course, picking between Houndour and Growlithe ends up being just as hard!
This is why I picked Turtwig when I got round to getting Platinum. I always picture the Giant turtle from that Zelda stage on Super Smash Bros. I imagine if I was allowed that I would neglect my bike in favour of lounging on Torterra's back, drinking tea and pruning his grass, whilst he ferried me about. In my mind we would have a falling out as I attempt to drive an umbrella into his back, you know, for sun protection. He would obviously take exception to that and I would have to treat him to a fancy meal before we could go back to first name terms. That's the dream anyway.