There is not such a thing as a "best team", Reborn forces you to make diverse teams for the variety of battles and terrains. While you can argue about what the best pokemon is, or the one that is usable in every terrain, it all comes to your style of playing, which can change even depending on the battle. For example, you would prefer an strong and fast team against Charlotte, but it's better to have setup sweepers in Terra's battle. There can't be a best team, since limiting a playtrough with only a team of six pokemon would seriously hinder their versatility.
PD: But please, please, please use a Terrain user, like Sylveon or Alakazam, it really helps out, ok bye... :]
Whoops, I've just forgot this "rule" by "disagreeing" with my response xD