Well, I agree that Arceus should be in, but I'll contribute anyways. Sounds like the kind of pointless fun that the Ubers forum needs.
First of all, the only Pokemon that can pull off a legitimate boosting sweep of this selection are Mewtwo, Manaphy, Excadrill, Groudon, and maybe Zekrom. I'll talk about the other three a little later.
Because the contest is to determine which of the Pokemon is most likely to pull off the sweep, Manaphy and Excadrill have to be called into question. Their dependence on weather makes them unlikely (relatively speaking) to endgame sweep, especially considering your opponent probably has a weather inducer in one of their 6 slots, and there's a decent chance that it's not yours. This leaves us with Mewtwo, Groudon, and Zekrom. Zekrom lacks the ability to boost its speed, which, compounded by its inherent lack of speed, means that it's probably not going to sweep. As for Mewtwo vs. Groudon, I'd say Mewtwo is superior; Groudon has to either run Double Dance with Earthquake + 1 coverage move or run Rock Polish with Earthquake and 2 coverage moves. With one coverage move, Groudon will always be countered by Giratina or Lugia, depending on the move. Even with two coverage moves, it can't hit everything for enough (Stone Edge+Dragon Claw is walled by Skarmory, Stone Edge+Fire Punch is walled by Giratina, Dragon Claw+Fire Punch is walled by Lugia). Mewtwo can hit everything for an enormous amount of damage with one setup move, and that's all it needs due to its inherent speed.
As for Ho-oh, Dialga, and Giratina-O, I'm torn. I'd love to say Ho-oh, but its dependence on weather and spin support hurt its chances. It's probably not Giratina-O (unless you're running a Tailwind set) just due to a lack of power and strong coverage. Dialga, while slow, hits hard and between Dragon Pulse, Fire Blast, and Thunder it can take down a lot of threats. Its bulk helps it as well, allowing it to run a status move like Thunder Wave in the third slot to take a hit and cripple the opponent.
Because of the reasons stated above, I'd say that for this scenario Mewtwo wins, and Giratina-O/Excadrill loses.