It's a tie for me with The Swimmer and From Here To Eternity.
FHTE because it's just a great movie all around. I actually don't think of it as a "Lancaster" movie, more of an ensemble. I would pick Judgement At Nuremberg for the same reason.
The Swimmer is definitely a Lancaster movie. I discovered it as a kid and we used to make fun of it because it's such a wacky concept. The plot, if you don't know it, involves a guy (Lancaster) "swimming" across the city to get home. He basically follows a path of swimming pools in people's yards that lead back to his house. He shows up, swims across somebody's pool, then goes to the next yard, and so on. He knows all the people that own the houses and as he chats and swims his way across the city, his history and their stories are revealed. High concept stuff.
But despite participating in the mockery of the film as a kid, to be cool with my buddies, I still knew then that it was quite the acting showcase. I've watched it again as an adult with a new appreciation, both for its handling of drama and, yes, for the wacky concept and late-60s groovy vibe.
Actually, looking at Lancaster's IMDb, there are quite a few films of his I haven't actually seen. Some are his most famous. For example, I haven't seen either of Elmer Gantry or Birdman of Alcatraz. I haven't seen The Train either! I gotta remedy that. When I do, my list of favourites may change.