Why Won't They Tell Us?
Hangar 18 (1980) is billed as a pseduo-based-on-reality "now the truth can be told" tale about a crashed UFO coverup. It isn't (naturally), but it does feature Darren McGavin going head-to-head with government toadie Robert Vaughn as the former tries to uncover the truth hidden by the latter. I don't need much more than that to make me a happy sci-fi viewer.
Click through for the film's full poster (painted art McGavin & Vaughn!), a TV spot, and the first segment of a KTMA-era MST3K episode featuring the film.