To JJones1:I think the BEST pokemon is an All-rounder like Salamence (Don't STFU me please, I am just answering a question and this is only an example so don't acctually train one, unless you can't help it.) with Brick Break/Earthquake, Aerial Ace, Flamethrower and Dragon Claw and with three of these, you minimise chances of losing, so here is a team I suggest: A mixed sweeper, a special sweeper and a CB physical sweeper. But, to be honest, THERE IS NO BEST POKEMON! THERE ARE AWESOME TEAMS BUT NO BEST POKEMON, ALL POKEMON HAVE WEAKNESSES, at least one, and at least 2x weakness (this is Kingdra BTW) and can be beaten with a wholesome STAB move or its weakness (No Sh....)
To PUreHeart: I think a better moveset for Metagross is Meteor Mash, Sludge Bomb, Earthquake and Explosion (adamant, Choice Band) but Agility works... I hate using it though, useless (IMO. Don't STFU me please).
To Shining Mew: I like Frontier, the problem is that is is quite hard to get Symbols. Beating the brains themselves is basic, as in ABC, but getting up to them is hard. IMO. I hate Palace Maven Spencer... The one Brain I can't beat (so far, Got three symbols and the only Brain I have gotten up to, and not won, is Spencer. Noland doesn't count because his team is ever changing) I am currently trying to beat the Pyramid.
To Those who asked for a team built for them: STFU. That is not good. You should really make your own teams. Make one and then get it rated by other members, they will help you, but I don't think anyone would build you a team from scratch. It is also unwritten law that all teams should be Copyright, just my Opinion though... I think it is against the rules to ask for teams built for you in serebii, I don't know though so correct me if I'm wrong.