Weird computer error stopped me from seeing my score, guessing around a 70% minimum. All fear Blissey!
Blissey @ Life Orb/Leftovers
Adamant 252 HP/252 Atk/4 Spe w/ 0 IV Def and SpD
~Power Swap
~Guard Swap
~Return
~Softboiled/Earthquake/Brick Break
Most counters to Blissey have high Defense (Tyranitar, Metagross, etc.). This set uses Guard Swap to give the opponent low defenses before Power Swapping to boost its Attack sky-high (for Blissey), opening them up for a sweep. With Softboiled, you can PP stall just about anything that walls this set, especially if you got Tyranitar defenses. Earthquake hits Rocks/Steels except Skarmory, Bronzong, and Aerodactyl. Brick Break has neutral coverage on them but is weaker against Metagross, Jirachi, and Heatran.
The choice between Leftovers and Life Orb depends on whether you want survivability or power. It's recommended to use Leftovers without Softboiled, and Life Orb with it. EV's/IV's create maximum defenses after a Power Swap while giving your opponent low defenses.