Band: Garchomp
Garchomp's 130 base attack combined with 102 base speed is much more menacing than the base 80 speed of Heracross, and whatever Tyranitar's is. Combined with two STAB attacks of 100 BP or more, CB Garchomp easily 2HKO's almost every pokemon sent in to wall it, not including Skarmory (though this can be changed by using Fire Blast). Earthquake and Outrage take pokemon like Swampert, Cresselia, Zapdos, Vaporeon, and many others right out of them game.
Scarf: Heatran
This shouldn't be a surprise to anyone. Heatran has a great SpAtk stat, and boasts a rather good movepool, despite having to use a Hidden Power as an Electric or Ice attack. Fire Blast is a great attack to make up for the loss of power compared to specs, and Heatran's Flash Fire makes this even more formidable. Heatran also has some great resistances, making switching in that much easier.
Specs: Lucario / Porygon-Z
Lucario: Aura Sphere is just a beastly move all in all. 2HKOing Blissey, and basically everything else in OU. Lucario also gets Dark Pulse or Shadow Ball, to complete the duo. Hidden Power Ice takes down Salamence, who is otherwise a decent counter to this set. Finally, we have the icing on the cake: Vacuum Wave. Vacuum Wave is a great priority move, coming off STAB and over 500 SpA. A great thing to know, is that so many, otherwise threatening pokemon, can be OHKO'd by Vacuum Wave. Lucario can come into both of Tyranitar and Weavile's STAB attacks, and threaten a OHKO. Heatran takes around 70% from Vacuum Wave. I don't even want to start listing the things VW hits neutral that take big damage from it.
Porygon-Z: This may seem like an odd choice, but it shouldn't. Tri Attack is a 160 BP attack coming from Porygon-Z's base 135 SpA, boosted to 607. Not to mention the 20% chance of status. Porygon-Z also has a great movepool to back up Tri Attack, including Ice Beam, Thunderbolt and Dark Pulse/Shadow Ball. The amount of pokemon 1-2HKO'd by this monster is huge. The reason I can't put Porygon-Z as the definate best specs user is because Blissey can still stop it (only to a certain extent though. Status, crits or just a weakened Blissey means that Porygon-Z wins).