Absolutely worst sci-fi movie of all time:
The first Starship Troopers. Let's just say I read the Heinlien masterpiece before watching that POS on videotape. Dig up my rant thread on Robotech.com to get the details (I'd link to it if I could, but Steve Yun, in his ultimate wisdom {and lack of sense of humor} decided to permaban me from that site). To say I didn't like the film is putting it mildly.
Although I can make a case for Plan 9 From Outer Space, but since it became a cult classic because Ed Wood went out of his way to make that movie as terrible as it was, I give it second place.