It's a choice between Totodile and Cyndaquil. Cyndaquil was my starter on my first Silver game, so if I choose Cyndaquil, it'll help relive the experience from before. But Totodile is currently my favorite out of the three starters.
I'll probably just choose Cyndaquil on my SoulSilver game, and trade over a Totodile from my HeartGold. Just because I'll go with the Fire starter in the first place, as I tend to do.