λ> pickMajority id [1]
1
λ> pickMajority id [1,2,2]
2
λ> pickMajority id [1,3,3,3,2,2]
3

pickMajority :: Ord b => (a -> b) -> [a] -> a
pickMajority f = head
               . head
               . sortBy (flip $ comparing length)
               . groupBy ((==) `on` f)
               . sortOn f