My submission for the fortnightly challenge. Entries for the current showdown occupy only the top ten (which are the only entries sampled for the results), with those for past showdowns listed in descending order of recency. The first of every selection has the showdown category in its notes. Movies can be reused in up to three later showdowns, so older categories may now contain fewer than ten entries due to some of their films having moved into a more recent selection. (The notes for a film will detail its past appearances.)
These are ranked, but in groups of ten(ish, as enumerated above), so the ranking feature wouldn't make much sense (and the ranking doesn't matter for the results either).
Since this page actually seems to have some eyes on it once in a while, I'll plug a list on which I've actually worked quite hard.