I remember one day hearing my father downstairs laughing his ass off. Went down to see what was so funny to find he had just discovered Mystery Science Theater 3000, and was losing it watching The Giant Gila Monster. I sat down to watch. I didn't really get it then, but I've been a fan ever since.
Mitchell, Manos, and Pod People are probably a three-way tie for me. Some of my underrated favorites are Girls Town, The Incredible Melting Man, The Giant Spider Invasion, Teen-Age Crime Wave, and Kitten With a Whip.
I think Giant Spider Invasion is the first one I ever saw, randomly showing on SciFi channel early one morning. I was dying from laughter, just laying on the floor gasping for breath from laughing so hard. Been a fan ever since!