So look for some homosexual or lesbian love story flick (I can't think of one) to finish in the top 5. And maybe more than one, if there are more than one to pick from.
The Freep thread about the Century's best comedies is here.
The first thing I noticed was that cross-dressing movies took the top two slots. I think you're right. Expect something weird to be very high. I'm thinking "Last Tango in Paris" would be a high, except I think it's a foreign film. According to some reviews I've read, it is actually a homosexual movie, but they chose Maria Schnieder to play Brando's lover because they knew a homosexual love movie wouldn't sell. My bet is that "Philadelphia" comes in somewhere.
As far as what I'd vote for, Somewhere in Time, Tender Mercies, Angel and the Badman, The Quiet Man, and It's a Wonderful Life.