Are we talking so bad they are good? Many of the 50’s drive in movie flics are just plain awful but so much fun to watch. A short list of my favorites.
The Wasp Woman
It! The terror from Beyond Space. (I just watched this one)
The Giant Gila Monster
The Killer Shrews…(make up! That dogs costume is slipping!)
Attack of The Crab Monsters
The Undertaker and his Pals
Just about everything by Roger Corman up to his Poe series.
Second the motion on “Motel Hell.”
I’m partial to films that cast Wolfman Jack as a fundie preacher.
Masters of Menace
That guy who was in the Sledge Hammer TV series as the leader of a biker gang. Their mechanic Gyspy (Jim Belushi) dies and they take his body cross country, parole officers giving chase. Cameos by all sorts of people (George Wendt, Dan Ackroyd, John Candy, etc).
Hell Comes to Frog Town?
** Masters of the Universe**
Cheese galore, but damned if I don’t catch it whenever it comes on cable. 
Grease 2.
Beastmaster.
Battle Beyond the Stars.
I’m sad a couple of people have nominated Tremors for this thread, Tremors was a GREAT movie!
** Masters of the Universe**
Cheese galore, but damned if I don’t catch it whenever it comes on cable.
I’ve mentioned this in another thread recently, and I’m certainly not proud of it, but here goes: Nothing But Trouble has a strange hold over me. I cannot not watch it if it’s on.
I second all the 50s sci-fi/horror flicks mentioned here, the ones that I’ve seen, anyway, which haven’t been many. 
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For all of you who mentioned horror movies, I recommend the nonfiction book Danse Macabre by Stephen King. It’s an interesting look at the tradition.
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Stop! or my mom will shoot!
I love that film, though its absolute crap. i bought it for 99p on video at blockbusters.
Super Troopers
Flash Gordon
etc
Dungeons and Dragons: The Movie is utterly awful in a wonderful way.
A few others:
Tarzan, the Ape Man (The Bo Derek one)
Invaders from Mars (the original version. Some people say this Mezies film is a classic. I don’t think they’ve seenm it in a while. The remake is pretty bad, too.)
** The Manster
Star Crystal** (They defeat the alien by converting it to Christianity!)
The Creature that Wasn’t Nice/Starship/The Naked Space (another Bruce Kimmel offering – “Alien” as a musical! With Cindy Williams! And Patrick McNee! And Leslie Nielsen!)
Them’s fightin’ words! “It!” is one of the overlooked gems of 1950s cheap film SF. “Alien” ripped off its plot, and jettisoned "It!'s logic and SF aspects for “Old Dark House” spookiness (although they had a better-looking Creature). Jerome Bixby, who wrote the screenplay for this and other neglected films (The Atomic Missile, Curse of the Faceless Man), and who wrote the story “It’s a Good Life” that has become Twilight Zone canon, desrves to be better remembered.
Red Dawn. A pretty bad movie, but I couldn’t help watching it over and over on HBO when I was in college.
Star Crash with Marjoe Gortner and Caroline Munro. Contains the greatest cheap out ever: “Hey, we’re trapped inside a planet that will explode in five seconds!” “Imperial Battleship … stop the flow of time!”
That was The World Is Not Enough. That chick who played Lois on Lois and Clark (Terri something) was the Bond babe in Tomorrow Never Dies.
And all I have to tell you is that I am the World’s Biggest Madonna Fan. Ergo, my list is pages long. 
Monster stomping is always fun, but give me a big-budget disaster movie every time. Just make sure the premise is ridiculous, the cast is famous enough to have known better than to sign up, and the script is totally, woodenly pretentious, just as long as the SFX crew was given free rein.
Any of the Airport sequels, but not the original pretty-decent original though. Airport '75, with the rescue pilot being lowered through the broken cockpit window of the 747, is a classic. The one with the sunken 747 and the one with the Concorde crash-landing on a ski slope are must-see’s, too.
Independence Day - President Bill Paxton’s inspirational speech before getting into his F/A-18 to attack the alien mother ship is a highlight of the genre.
The Towering Inferno - Possible O.J. Simpson’s finest acting work, even including his Barbara Walters interview.
The Poseidon Adventure - you’d never think Gene Hackman went on to a decent career after that.
Goliath Awaits - another forgotten jewel with an all-star cast. A sunken luxury liner provides a survival habitat for its trapped passengers for 40 years, letting them form their own society complete with Morlocks.
I gottta go with Road House as well.
You want bad disaster movies? Try The Swarm. An all-star cast in a no-star movie.
And the musical Lost Horizon is appalling.
Leningrad Cowboys Go America. I think this movie’s hilarious, but I appear to be one of the few.
I agree with skeptic_ev about Nothing But Trouble. It’s one those films that has a strange appeal for some people, but everyone else hates.
I think another movie that deserves a mention in this thread is Dark Star.
from the 1980s, so bad it’s good: Swamp Thing - with Louis Jourdan and Adrienne Barbeau. (!)
Still in some theatres, so bad it’s good: Once Upon A Time In Mexico. yep, it’s my guilty pleasure.