Perhaps it was the C++ programmer in me who immediately thought about maximum/minimum integers as the initial values for these functions.

maxList :: [Int] -> Int
maxList xs = foldl maxop (minBound::Int) xs
    where maxop x y = if (x < y) then y else x

maxList' xs = maxop xs minBound::Int
    where maxop (x:xs) m = if (x < m)
                           then maxop xs m
                           else maxop xs x
          maxop [] m = m

minList :: [Int] -> Int
minList xs = foldl minop (maxBound::Int) xs
    where minop x y = if (x < y) then x else y

minList' xs = minop xs maxBound::Int
    where minop (x:xs) m = if (x > m)
                           then minop xs m
                           else minop xs x
          minop [] m = m