def most_frequent(data):
    import collections
    return sorted(collections.Counter(data).items(), key=lambda item:item[1], reverse=True)[0][0]

if __name__ == '__main__':
    #These "asserts" using only for self-checking and not necessary for auto-testing
    print('Example:')
    print(most_frequent([
        'a', 'b', 'c', 
        'a', 'b',
        'a'
    ]))
    
    assert most_frequent([
        'a', 'b', 'c', 
        'a', 'b',
        'a'
    ]) == 'a'

    assert most_frequent(['a', 'a', 'bi', 'bi', 'bi']) == 'bi'
    print('Done')