maxByVal

maxByVal :: Ord a => Map.Map k a -> Maybe (k, v)
maxByVal m = Map.foldrWithKey (Just . f) Nothing m
  where f :: Maybe (k, a) -> (k, a)
        f k v Nothing               = (k,v)
        f k v (Just (maxKey, maxVal))
          | v <= maxVal = (maxKey, maxVal)
          | otherwise   = (k, v)