For me, it depends on the generation. I always pick Charmander out of the Kanto starters, but may choose one of the others if I feel like playing as a different one for a change. When I choose from the Hoenn and Johto starters, I always have trouble deciding which ones to have. When the Sinnoh and Unova starters were first revealed, I had trouble deciding which to have until their evolutions were revealed. However, when the trailer for X/Y was first revealed, I already knew right away that Fennekin was the one, even after seeing its evolutions. As for Sun/Moon, I'm currently torn between Rowlett and Litten and if those leaked evolutions turn out to be real, I still won't be able to decide.