Almost a cliché choice, but mine is Plan 9 from Outer Space. It has it all - grade-school special effects, ridiculous dialogue ("This man has been murdered. And one thing is for sure - SOMEONE is responsible!"), an actor who died early in filming replaced by a real-life chiropractor who covered his face with a cape to not make it obvious he wasn't the real actor, sets made from stuff bought at Sears (the shower curtain that divides the spaceship areas), scenes change from day to night and back to day unexplainedly.
Close second is Robot Monster. The alien is someone in a gorilla suit wearing a deep-sea diver helmet. Another ridiculous plot, yet it was John Carpenter's favorite movie as a kid.