For me, it was Emboar. I wasn't planning on using it too much at first, but since it was the only Pokémon in my team that wasn't underleveled, I allowed it to lead most of my battles. He's managed to win all of them. Scrafty was also quite important to me, because it had the perfect mixture of ability and nature (Moxie and Adamant, respectively). This Pokémon was able to defeat a couple of gym leaders completely by itself, haha.