I have compiled a list of all of the OU pokemon, and how they fare against a CB Head Smash from Aggron, here are the results.
Red=OHKO
Orange=2HKO
Green=3HKO
Black=4HKO
CB Aggron Calcs
Head Smash
Max/Max Aerodactyl: (185.16% - 218.13%)
Max/Max Alakazam: (129.94% - 153.18%)
4 HP Azelf: (164.38% - 193.49%)
with Reflect: (82.19% - 96.92%) With SR and Reflect there’s still a 58.97% chance to OHKO
Max/Max Blissey: (90.90% - 107.14%) OHKO 41.03% of the time, always with SR
248 HP Breloom: (66.87% - 78.64%) Possible OHKO with Spikes and SR, 2HKO otherwise
4 HP: (82.44% - 96.95%) 20.51% to OHKO with SR
Max/Max Bronzong: (34.32% - 40.53%) 3HKO, 2HKO with enough prior damage
252/220 Celebi: (72.28% - 85.40%) Possible OHKO with Spikes and SR, otherwise 2HKO
Max/Max Cresselia: (51.35% - 60.36%) 88.30% to 2HKO with Leftovers
Max/Max Dragonite: (100%+)
Max/Max Dusknoir: (71.43% - 84.01%) 2HKO
Electivire:(100%+)
252/160 Calm Empoleon: (45.16% - 53.23%) Possible 2HKO with SR
252/176 Neutral Flygon: (48.35% - 56.87%) Easy 2HK0 with SR, offensive ones have no chance
Max/Max Forretress: (57.63% - 68.08%) Easy 2HKO
Max HP: (75.71% - 89.27%) Has a 7.69% to be KO’d with SR
Gengar: (100%+)
252/40 Gliscor: (80.79% - 95.20%) 53.85% chance to OHKO with SR, 2HKO otherwise
Max/Max Gyarados: (101.02% - 119.29%) Not even an Intimidate phases Aggron!
252 HP Heatran: (88.08% - 104.15%) Always OHKO with SR
4 HP: (104.94% - 124.07%) OHKO
Heracross: (100%+)
252/168 Hippowdon: (32.14% - 37.86%) Possible 3HKO with SR
Infernape: (100%+)
252 HP Jirachi: (44.31% - 52.23%) 74.88% Chance to 2HKO
Jolteon: (100%+)
252/68 Adamant Kingdra: (98.31% - 116.10%) 87.18% chance to KO, SR finishes it
252 HP Latias: (107.42% - 126.92%) OHKO
4 HP Lucario: (42.55% - 50.00%)Possible 2HKO if it has used LO already, 3HKO otherwise
252 HP Machamp: 56.25% - 66.15%) Easy 2HKO
172 HP Magnezone: (52.30% - 61.51%) Always 2HKO with SR
124/64 Adamant Mamoswine: (102.81% - 121.17%)
252/176 Metagross: (29.67% - 35.16%) Great switch into Aggron, not even a 3HKO
252 HP Metagross: (39.29% - 46.15%) Need Spikes and SR for 2HKO, otherwise 3HKO
Ninjask: (1019.01% - 1201.52%) No chance lol
Porygon-Z: (100%+)
244/184 Rhyperior: (28.47% - 33.80%) 4HKO at best even with SR
252/168 Rotom-A: (78.29% - 92.76%) 33.33% chance to OHKO with SR, 2HKO otherwise
Max/Max Salamence: (100%+)
248 HP Scizor: (100%+)
Max/Max Skarmory: (61.08% - 72.16%) Easy 2HKO
Smeargle: (100%+)
168/120 Neutral Snorlax: (87.78% - 103.73%) Always an OHKO with SR
136/156 Neutral Starmie: (117.29% - 138.31%) OHKO
Max/Max Suicune: (58.17% - 68.56%) Easy 2HKO
Max/Max Swampert: (34.16% - 40.35%) Possible 2HKO with Spikes, otherwise 3HKO
252/120 Neutral Tentacruel: (118.68% - 139.56%) OHKO
Togekiss: (100+)
252 HP Tyranitar: (81.93% - 96.53%) 58.97% chance to OHKO with SR
252 HP Umbreon: (84.01% - 98.98%)74.36% OHKO chance with SR
188/252 Vaporeon: (78.57% - 92.63%) 33.33% OHKO with SR
Weavile: (100+)
248/228 Zapdos: (152.48% - 180.16%) OHKO
Results
-30 of the top 49 most used pokemon are OHKO'd by Head Smash
-14 of them are 2HKO'd
-5 of them are 3HKO'd
-The only 2 that are 4HKO'd, Rhyperior and Metagross, have to run super defensive sets in order to not be 3HKO'd, thus losing offensive power.
Just something I wanted to share, and hopefully convince some people of how powerful the CB set really is!