Cresselia @ Leftovers | Levitate
Relaxed | 248 HP / 232 Def / 28 SpA | IVs: 2 Atk / 30 SpA / 2 Spe
Trick Room | Hidden Power [Fire] | Ice Beam | Moonlight
Heatran @ Choice Specs | Flash Fire
Quiet | 248 HP / 8 Def / 252 SpA | IVs: 3 Atk / 2 Spe
Eruption | Earth Power | Hidden Power [Ice] | Flamethrower
Okay. The premise of this core: SpecsTran with Eruption is definitely up there in terms of sheer power. Let's abuse it. The problem: it's only available with a Quiet nature. Problem 2: Quiet Heatran is pretty much set-up fodder for many relevant threats in OU, ranging from Sheer Force RP Landorus to Salamence. The solution: run Trick Room. This simple move (haha, yea right...) turns the speed-based metagame over on its head, allowing Heatran to get the better of what would normally give it problems. Common switch-ins, such as Dragonite, Garchomp, Keldeo, Terrakion, Chansey, and Blissey, are decimated by the power behind Eruption underneath the sun, and the only mons immune to Eruption (opposing Heatran and Chandelure are the only ones on the OU Viability Ranking thread that boast this ability) are OHKOed by Earth Power, and Hidden Power Ice is used to conserve Eruption's PP against teams that stock up on Ice-weak Pokemon.
Cresselia is no less important than Heatran, and arguably is even more important, since she's incredibly bulky on the physical side. Even better, she resists almost all of Heatran's weaknesses, and can set Trick Room up as well. With Moonlight, Cresselia has a semi-reliable method of recovery, which comes in handy when dealing with teams that pack both Landorus and Keldeo. Trick Room turns the speed-based OU metagame over on its head, and the two attacking moves given provide super-effective coverage against the majority of OU. Psyshock can be used over one of the attacking moves to hit fighters for super-effective damage, but that comes at the cost of either hitting dragons hard or incinerating Scizor and Ferrothorn.
In regards to teammates, Ninetales is the best partner for the duo, since it brings eternal sunlight, powering up Heatran's Eruption and enhancing the recovery Cresselia's Moonlight brings to her. Stealth Rock setters such as Landorus-T and Forretress help this core immensely, for obvious reasons. A Chlorophyll mon such as Venusaur is good to have as a late-game sweeper and Keldeo / Terrakion check.