Movie and Game showdowns are done all the time. Also, our latest showdowns of each were ran really well and came out with fine results. If you want to do Movie, do a sepcific genre, like Best Western or Best Comedy. Or Best FPS or Best RPG with games.
Music showdowns lead to prick-filled bash topics.
I don't think I've ever seen a TV showdown here, that I can recall. But I would suggest, again, limitting it to a certain genre like Best SciFI or Best Sitcom.
..Just my two cents.