I think I'm going to agree with Focus in that Dialga has the best chance of beating both. The next best contestant is Zekrom spamming Choice Banded Fusion Bolts (Bolt Strike might miss), but that move doesn't always OHKO maximum defense blissey who can then use counter and kill Zekrom. Choice Banded Bolt Strike also has an extremely low chance of not OHKOing so the same thing happens there.
Sub Dialga can fry Skarmory. The only way the pair can win against Dialga is foddering Skarmory to Taunt Dialga, and then going to Blissey so that it can use Counter / spam Seismic Toss or Focus Blast should it decide to Thunderbolt to try to get rid of Taunt.
Rest Sleep Talk Dialga might have a better chance but idk. Dialga's poison immunity is the reason why I selected it (it can switch into Toxic and not be dead), the other contestant's risk getting haxed by misses.
Also I'd much rather perfer to use Flamethrower > Thunderbolt since we have weather control.
Technically its impossible to beat Blissey + Skarm if we factor they hold Brightpowder and fluke out. Its also impossible to beat if we assume they paralyze the incoming switch in and seismic toss us to death because of parahax. That and Minimze Blissey.
If we're going to think of the contestant that has the worst chance I would nominate Manaphy who basically gets stomped all over by Psych Up / CM Blissey that uses Thunder. Rest Talk users loose if there are no hazards up since the enemy can switch in and out without really caring about what happens and pp stall the rest talk user out of pp. If they are able to predict your every move giratina-o will be unable to do anything.
If we assume that the switch-in comes in safely, then Giratina-o has the best shot with a set consisting of Rest Dragon Tail CM Aura Sphere since even minimize Blissey can't beat that. I don't know how well that fare's against SD Skarmory though... using Rain Thunder might also be a decent idea that works over Aura Sphere. But minimize + CM Blissey wins since we assume that Blissey has better luck and gets critical hit Ice Beams.
Also I'm going to render Forretress useless by saying Skarmory / Blissey can run HP Fire or Flamethrower if its in the sun, and if its not Blissey's going to render it useless with an Ice Beam freezing X_X. Yes the chances of this happening are abyssmal but we are supposed to assume every outcome right?
(15:32) Dusk [DW Uber]: Actually Deoxys-a is not the frailest uber
(15:32) Dusk [DW Uber]:Its Carvanha
(15:33) X-Spin: O.O What are its bases?
(15:33) Dusk [DW Uber]: 45/20/20 so close to matching Deo-a's 50/20/20
(15:33) X-Spin: That's enough to wall Specs Kyogre!
Great Sage is the best.