This is what I'm going to be using for Gen 6:
Kangaskhan @ Kangaskhanite
Nature: Jolly (+Spd, -Spatk)
EVs: 252 Atk/ 252 Spd/ 4 HP
Return (STAB damage, 16 types of coverage)
Fake-Out (For the first turn it gets sent out, you can mega evolve and apply your mega stats so that your speed and attack damage increases)
Earthquake (Coverage for the Steel and Rock that you can't cover with Return)
Sucker Punch (Priority move and helps dealing with the Gengars and any other Ghost/Psychics you find)
Note that since Mega Kangaskhan attacks twice with each move, I'd say this it's going to be one of the new OU's.
Well, this is just my moveset anyway :D.
Edit: Okay, so while battling random people online, I've noticed a few things:
1) A bunch of these people love using Yveltal/Xerneas.
2) Be prepared to sacrifice a wall to keep Kangaskhan safe from harm.
Anyway, with those two legendaries, I tend to Fake out either one and then swap to a SDef wall like my Goodra or Sylveon (This is mainly for Yveltal). If Xerneas was to kill one of them, I already gave it some Fake out damage. The next turn Kangaskhan appears would give it the second Fake out and then a Return for the finishing blow. Yveltal is nothing special against Sylveon so there isn't much to worry about (Dem Oblivion Wing users).
Also, went up against a couple Mega Garchomps as well. Two Fake outs and a Return will kill it (using this speed set, you'll be faster than a 92 base speed Mega Garchomp). You also survive earthquakes from this badboy. I haven't seen an outrage Garchomp in mega yet, but I hope to test the damage out for that shortly.