The most common ability in the 6th generation is actually "Competitive" with eight Pokemon. Those Pokemon with this "super rare" ability are Jigglypuff and his/her whole evolution line (three Pokemon), Gothita and his/her whole evolution line (three Pokemon) and lastly we have Milotic. We also have female Meowstic with the ability as a hidden one.
The most common ability ever is a tie between "Levitate" and "Swift Swim".
Sources: Pokemondb Link --> Competitive Pokemon Saw what I did there? XD
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