Better late than never for me as well.

Grass: Serperior, though Sceptile isn't too far behind; Contrary is the major selling point here, of course.
Fire: Charizard; when Charizard mega evolves into Mega Charizard Y, it packs some serious, and I mean serious heat.
Water: Greninja. Protean is definitely cooler IMO than Torrent in this case, although Battle Bond and its subsequent transformation into Ash-Greninja after it directly KOs an opponent make it even better.