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Some Calculations
Offensive [Rapid Spin + U-Turn + Earthquake + Head Smash, 252+](Not the best set, but the most offensive)
Bronzong (OU Tank)   Head Smash   12.72 - 15.08%   U-turn   12.13 - 14.49%
Bronzong (OU Dual Screens)   Head Smash   14.79 - 17.45%   U-turn   13.9 - 16.56%
Breloom (OU SubSeed)   Earthquake   14.68 - 17.5%   Head Smash   14.68 - 17.5%
Cresselia (OU Defensive)   Head Smash   16.47 - 19.41%   U-turn   15.34 - 18.51%
Porygon2 (OU Defensive Duck)   Earthquake   16.84 - 20.05%   Head Smash   16.57 - 19.78%
Tangrowth (OU Physically Defensive)   Head Smash   17.36 - 20.59%   U-turn   16.87 - 19.85%
Hippowdon (OU Physical Wall)   Earthquake   17.38 - 20.71%   Head Smash   8.57 - 10.23%
Cresselia (OU Dual Screens)   Head Smash   18.05 - 21.44%   U-turn   17.15 - 20.31%
Landorus-T (OU Offensive Pivot [Intimidate])   Head Smash   18.11 - 21.52%   U-turn   4.19 - 4.98%
Suicune (OU CroCune)   Earthquake   18.56 - 21.78%   Head Smash   18.31 - 21.78%
Forretress (OU Rapid Spin)   Earthquake   18.64 - 22.03%   Head Smash   18.36 - 21.75%
Hitmontop (OU Rapid Spin [Intimidate])   Earthquake   18.75 - 22.03%   Head Smash   9.21 - 10.85%
Terrakion (OU Double Dancer [Air Balloon])   Head Smash   19.13 - 22.53%   U-turn   8.95 - 10.49%
Virizion (OU Dual Screens)   Earthquake   19.17 - 22.53%   Head Smash   18.91 - 22.53%
Donphan (OU Rapid Spin)   Earthquake   19.01 - 22.65%   Head Smash   9.37 - 11.19%
Slowbro (OU Physical Wall)   Earthquake   19.28 - 23.09%   Head Smash   19.54 - 23.09%
Skarmory (OU Physically Defensive)   Head Smash   19.87 - 23.54%   U-turn   2.14 - 2.75%
Gliscor (OU Defensive)   Head Smash   19.88 - 23.57%   U-turn   4.82 - 5.68%
Mew (OU Support)   Earthquake   20.79 - 24.5%   Head Smash   20.79 - 24.5%
Mew (OU Baton Pass)   Earthquake   21.03 - 25.24%   Head Smash   21.28 - 25.24%
Chansey (OU Wish)   Earthquake   21.44 - 25.28%   Head Smash   21.3 - 25.14%
Wobbuffet (OU Speedy)   Earthquake   21.88 - 25.91%   Head Smash   21.68 - 25.71%
Wobbuffet (OU Oh No, It's Wobbuffet! (Bulky))   Earthquake   22.15 - 26.13%   Head Smash   21.96 - 25.94%
Porygon2 (OU Trick Room)   Earthquake   22.45 - 26.47%   Head Smash   22.19 - 26.2%
Ferrothorn (OU Standard)   Earthquake   22.44 - 26.7%   Head Smash   11.07 - 13.06%
Forretress (OU Custap Lead)   Earthquake   22.6 - 26.71%   Head Smash   22.26 - 26.36%
Virizion (OU Calm Mind)   Earthquake   22.83 - 26.85%   Head Smash   22.83 - 26.85%
Garchomp (OU Support)   Earthquake   23.09 - 27.38%   Head Smash   11.42 - 13.57%
Quagsire (OU Physically Defensive)   Earthquake   23.09 - 27.41%   Head Smash   11.42 - 13.45%
Reuniclus (OU Calm Mind)   Earthquake   23.34 - 27.59%   Head Smash   23.11 - 27.35%
Gliscor (OU SubPass)   Head Smash   23.57 - 27.84%   U-turn   5.39 - 6.53%
Hippowdon (OU Mixed Wall)   Earthquake   23.57 - 27.85%   Head Smash   11.66 - 13.8%
Vaporeon (OU Wish Support)   Earthquake   23.97 - 28.5%   Head Smash   24.19 - 28.5%
Landorus-T (OU Double Booster [Intimidate])   Head Smash   25.07 - 29.6%   U-turn   5.74 - 6.94%
Landorus-T (OU Choice Scarf [Intimidate])   Head Smash   25.38 - 29.96%   U-turn   5.81 - 7.03%
Skarmory (OU Specially Defensive)   Head Smash   25.38 - 29.96%   U-turn   2.75 - 3.36%
Jellicent (OU Utility Counter)   Earthquake   25.31 - 30.02%   Head Smash   25.31 - 30.02%
Virizion (OU Work Up)   Earthquake   25.38 - 30.03%   Head Smash   25.38 - 30.03%
Ferrothorn (OU Choice Band)   Earthquake   25.64 - 30.19%   Head Smash   12.82 - 15.09%
Politoed (OU Defensive)   Earthquake   25.78 - 30.46%   Head Smash   25.52 - 30.2%
Gothitelle (OU Calm Mind)   Earthquake   25.58 - 30.52%   Head Smash   25.58 - 30.23%
Landorus-T (OU Gravity [Intimidate])   Head Smash   25.93 - 30.62%   U-turn   5.93 - 7.18%
Breloom (OU SubPunch)   Earthquake   25.75 - 30.68%   Head Smash   25.75 - 30.3%
Breloom (OU Swords Dance)   Earthquake   25.95 - 30.91%   Head Smash   25.95 - 30.53%
Slowbro (OU Offensive Trick Room)   Earthquake   26.22 - 31.1%   Head Smash   26.22 - 31.1%
Golurk (OU Support)   Earthquake   26.7 - 31.41%   Head Smash   13.35 - 15.7%
Slowbro (OU Choice Specs)   Earthquake   26.64 - 31.47%   Head Smash   26.39 - 31.21%
Landorus-T (OU Offensive Pivot)   Head Smash   27.03 - 32.02%   U-turn   6.29 - 7.61%
Tangrowth (OU Chlorophyll Sweeper)   Head Smash   27.56 - 32.55%   U-turn   25.8 - 30.49%
Vaporeon (OU Baton Pass)   Earthquake   27.48 - 32.68%   Head Smash   27.7 - 32.68%
Slowking (OU Tank)   Earthquake   27.66 - 32.74%   Head Smash   27.66 - 32.74%
Hitmontop (OU Rapid Spin)   Earthquake   27.63 - 32.89%   Head Smash   13.81 - 16.44%
Whimsicott (OU Prankster)   Head Smash   27.77 - 33.02%   U-turn   26.54 - 31.48%
Dragonite (OU Substitute + Dragon Dance [Multiscale])   Head Smash   28.49 - 33.67%   U-turn   3.1 - 3.88%
Shaymin (OU Bulky Attacker)   Head Smash   28.46 - 33.75%   U-turn   26.7 - 31.73%
Conkeldurr (OU Substitute + Focus Punch)   Earthquake   28.5 - 34.05%   Head Smash   14.25 - 16.9%
Scrafty (OU Bulk Up)   Earthquake   30.53 - 35.92%   Head Smash   14.97 - 17.66%
Dragonite (OU Multiscale Shuffler [Multiscale])   Head Smash   30.56 - 36.01%   U-turn   3.62 - 4.14%
Dragonite (OU Tank [Multiscale])   Head Smash   30.82 - 36.26%   U-turn   3.62 - 4.14%
Conkeldurr (OU Status Orb)   Earthquake   30.8 - 36.81%   Head Smash   15.4 - 18.27%

Remember that Head Smash lets it take massive damage, so it won't be able to tank well anymore