I was a bit hard for me to choose, since the Gen III starters are my favorite generation of starters, but I've decided on Treecko. As much as I like Blaziken, there was the whole thing with the Torchic+Blazikenite in X/Y, and it's the starter I've used the most in Hoenn.
As for Swampert, I'm just about to finish a Water monotype run on my Ruby, so using it again so soon might not be so fun. It's been a while since I've used a Sceptile anyway, and its Mega is pretty nice. I suppose that's a bit unfair to Team Aqua, but oh well....
....I'm so mean to Team Magma and Team Aqua.