Return the list of tags ranked from most to least popular.

def _rank_tags(tagged_paired_aligns):
    '''Return the list of tags ranked from most to least popular.'''
    tag_count_dict = defaultdict(int)
    for paired_align in tagged_paired_aligns:
        tag_count_dict[paired_align.umt] += 1
    tags_by_count = utils.sort_dict(tag_count_dict)
    ranked_tags = [tag_count[0] for tag_count in tags_by_count]
    return ranked_tags