from collections import Counter

def checkio(text):

    count = Counter( c for c in text.lower() if c in "abcdefghijklmnopqrstuvwxyz" )
    
    c , n = min( count.items() , key = lambda t : ( -t[1] , t[0] ) )
    
    return c
    
    

if __name__ == '__main__':
    #These "asserts" using only for self-checking and not necessary for auto-testing
    assert checkio("Hello World!") == "l", "Hello test"
    assert checkio("How do you do?") == "o", "O is most wanted"
    assert checkio("One") == "e", "All letter only once."
    assert checkio("Oops!") == "o", "Don't forget about lower case."
    assert checkio("AAaooo!!!!") == "a", "Only letters."
    assert checkio("abe") == "a", "The First."
    print("Start the long test")
    assert checkio("a" * 9000 + "b" * 1000) == "a", "Long."
    print("The local tests are done.")

Create most-wanted-letter.py