Also, for the 'Aaaooo!!!!' test, it is working properly in the 'try it' section but not in the 'check and run' section.

def checkio(text):
    text = text.lower()
    mwl = {}
    for letter in text:
        if letter not in mwl:
            mwl[letter] = 1
        else:
            mwl[letter] += 1

    for letter in sorted(mwl, key = mwl.get, reverse = True):
        if letter.isalpha():
            letter = sorted(list(letter))
            return letter[0]