I propose six films:
Scrooge. This musical adaptation of A Christmas Carol has great music, a fantastic performance from Albert Finney, and is an incredibly entertaining film. If it was made a decade earlier, it would be a classic. Now it's a footnote.
Targets. The first film from Peter Bogdonovich, it stars Boris Karloff as an aging horror star (basically, he plays himself) who has to deal with the real life horror of a serial killer/sniper.
The Conversation. You have Gene Hackman. You have Francis Ford Coppola. You have one of the best explorations of paranoia ever filmed. And, while incredibly appreciated by film critics, it's been forgotten by most of the public. An absolute shame.
The Fisher King. Directed by Terry Gilliam, it stars Jeff Bridges and Robin Williams and I was lucky enough to meet Mr. Gilliam last year and tell him how much the film meant to me. It is fantastic and mostly forgotten.
The Ruling Class. Peter O'Toole as Jesus Christ AND Jack the Ripper. And it's one of the most damning indictments of British culture ever. It's a dark comedy so dark it borders on the ultraviolet. Did I also mention it was a musical?
The Ninth Configuration. The follow-up to The Exorcist from William Peter Blatty, this film is MASH in a military insane asylum, with some too on-the-nose Jesus metaphors. Still, underrated.