Really? What did you hate about Rin? I can understand why someone wouldn't like it, but I don't see how it could be the worst anime you've ever seen.
My favourite anime is Monster as well.
My least favourite anime is... I'm not sure. Probably Girls Bravo. I suffered through a few episodes with my friend and nearly died from its overall crapiness. The premise alone should have been a warning.
edit*
genre wise - my favourite is thrillers, least favourite is ecchi