The first time I played each gen, these are what I chose:
Gen 1: Charmander
Gen 2: Totodile
Gen 3: Torchic
Gen 4: Piplup
Gen 5: Snivy
I love the final evos of all of these. I also love the fact that Snivy is so sassy in the anime, or at least Ash's Snivy. Generally, though, what I do with each gen is choose a different starter for each main series game in the generation, such as choosing Piplup in Diamond, Chimchar in Pearl, and Turtwig in Platinum.