I don't really think there's a "best" starter. I think they're all good in their own regards. That said, I guess if I had to pick the best, see below.
Fire - Charizard, Blaziken, or Infernape
I think all three of these could be the "best" Fire type starter. Charizard and Blaziken have their Mega Evolutions, and Infernape just hits hard and fast. The 4x Rock weakness hurts Charizard, but Mega Charizard X takes that away.
Water - Feraligatr or Greninja
Feraligatr for Sheer Force + Life Orb and Greninja because Protean. Don't need to say anything else there.
Grass - Venusaur or Sceptile
Venusaur's kinda tanky, and Sceptile's Mega Evolution makes it seem pretty rad.