I would replace Minimize on Blissey with Aromatherapy (breed with Roselia) or Heal Bell (breed with Snubbul I think), at least for the Pyramid. Solid status curing for your entire team is incredibly helpful in the Pyramid, getting rid of burns, paralysis, and toxic without having to rely on berries you find. In the other facilities it is less useful I think, mainly because 3vs3 (or 4vs4 in tower doubles) is just too fast-paced to really have much use for clerics.
Anyways, alongside Blissey I would go with a physical powerhouse, something that can take and dish out physical hits well. Metagross is the best in my opinion (apart from the banned Groudon), getting a fantastic defensive typing, beastly defence, and enough attack to make good use of the turns it gets as well. The main choice in Metagross is Choice or no Choice:
Metagross @ CB
Adamant nature
252 Atk, some speed, rest in HP and Def (not sure on best spread, it also depends on your IV's)
-Meteor Mash
-Earthquake
-Rock Slide / Explosion
-Shadow Ball / Explosion
Shadow Ball is unusual on Metagross but it hits some key pokemon hard. It is your best weapon against Slowbro, Slowking, and Starmie, and it is more accurate than Meteor Mash against Alakazam, Gengar, and Dusclops. Rock Slide is more conventional but doesn't hit that much. It hits Shedinja (only used by Pike Queen), Charizard, Moltres, Zapdos, Gyarados, and Mantine good, and for Charizard and Moltres you should switch anyways unless you know Charizard to be purely physical. Explosion is useful against last pokémon and tends to destroy everything. CB Metagross hits seriously hard, OHKOing stuff like Tyranitar even with Earthquake, and 2KOing Slowbro and Slowking easily with Shadow Ball. I think its EQ even 2KO's Milotic and Suicune, but I'm not sure of that. It does OHKO some AI Metagross if you have good IV's.
Metagross @ Leftovers
Adamant nature
252 Atk, rest in HP, Def, and Speed
-Meteor Mash
-Earthquake
-Agility
-Light Screen / Rock Slide / Shadow Ball
Agility Metagross outruns mot of the frontier after a single Agility, and can switch between its moves. It's fairly straightforward
Metagross tends to just smash through physical walls except Swampert and Suicune (sometimes). Skarmory, Regirock, Aggron, Metagross, Snorlax, and Registeel tend to crumble under Metagross. Snorlax is a bit difficult, it depends on how well it Curses, but in general repeatedly hitting it with Meteor Mash over and over again eventually beats it because of MM attack boost.
Finally I'd go with a special attacker so you have offensive presence if Metagross goes down to an unlucky counter or w/e. Latios works well in that regard. Starmie also works well. Starmie is weaker but gets STAB Surf which is better than STAB Dragon Claw imo, and Starmie can breed, but Latios simply has better stats.