I'd probably say Priscilla, even though it's more about drag queens.
And how prominent does the TS theme have to be? Does Ace Ventura count? :D What about Austin Powers? Or Silence of the Lambs? Ya know, in Nighthawks, Stallone is in a skirt in the first scene and in a nightie in the last scene. Shallow Hal had an awfully manly tranny.
Princesa,
Beautiful Boxer,
Wild Side and
Normal are all good movies, but not things many people would want to see more than once.
And why doesn't anybody ever mention Dog Day Afternoon? Or
Dressed to Kill? Or
Orlando?
The Iron Ladies is supposed to be really funny.