The best girl is Hamster girl. She still stands apart as having already been, perhaps not in love, but 'into' him even before the series started. Hamster girl's feelings are 100% Gero, without even a little push to help. He could have married her without aid, well, if he wasn't too weak to survive her arc.
But at this rate I can't imagine he'll actually marry one of them. There'll either be a yaoi ending, a 'not actually a boy tee hee' ending, something dumb and open ended, or a harem.