Personally, here are some of my recommendations:
Greninja (with Protean) for obvious reasons.
Absol because with 150 Attack and Magic Bounce on Mega Absol, the beast is basically unstoppable.
Hydregion because its stat total of 600 and its diverse movepool (in Gen5 at least) makes it be able to cover all of its weaknesses.
Tyranitar for same reasons as Hydregion.
Scrafty (with Moxie) because once you kill one Pokemon with a Dark/Fighting moveset like Scrafty's, it becomes a great sweeper with attack boost from Moxie.
Malamar (with Infiltrator/Contrary) because it counters Bug types with Pluck, making its only weak point Fairy type Pokemon. Plus, Contrary can surprise trainers that lower your defense/attack stats, and if you have Infiltrator you can just use Topsy-Turvy to make Baton-Pass teams weak.
Bisharp (with Defiant)because with 125 Base Attack + Swords Dance + Defiant, Bisharp can easily OHKO almost all Pokemon with Night Slash or Iron Head.
Umbreon is a great mixed wall with 110/130 Defense stats, and a decent 95 HP and a lovely movepool.
Zoroark is a great physical Dark Type with access to Illusion and Substitute, and can be used with mindgames by nicknaming it the name of your last unfainted Pokemon. 105 Atk and Speed and 125 SpAtk can't do you wrong, especially with a couple of Hone Claws/Nasty Plots