I've always answered "Hudson Hawk" to this question. "Army of Darkness" is a fairly good answer as well. I see lots of people mentioning "Big Trouble in Little China" and I actually disagree there. It has its cheesy and silly moments, but I consider BTiLC actually to be a good movie. Not in any way "bad movie that I just enjoy for camp fun." It's a really clever deconstruction of the classic hero story. Because its seen from the viewpoint of the main character not being the hero, but his comedy sidekick, it does go a little over the top with the silly. But I can't really consider it "so bad it's fun" because I just think its damn good.
Someone mentioned "Bubba Ho-Tep." Same problem, except one thing. It's back-of-the-box blurb, 1 sentence synopsis may be silly. But the basic premise is the only thing "silly" about Bubba Ho-Tep. The actual run-time of Bubba Ho-Tep is nonstop dark comedy and bleak, oppressive, and confined atmosphere. I went into it expecting goofy and campy, and the second it was over I was headed to the library to check out every book by Joe R. Lansdale I could find. Heavy, DARK stuff, even the comedy. Going into Bubba Ho-Tep expecting silly camp is like watching Magica Madoka's first pair of episodes and expecting it to continue on as "this is as twee as Sailor Moon." It's THAT much of a shock to your expectations.
"Shoot Em Up?" Ehhh, I can't argue that it was bad. It was realllllly bad. It was so bad I couldn't even "ironically" enjoy it. The shootouts were silly, but the script and the dialogue were SO bad that a running commentary by Mike Nelson couldn't have kept Tom Servo's brains off the wall had they been forced to watch it. Between "Shoot em Up" and "Monster Man" I'm surprised the moron directing isn't ACTUALLY Clayton Forrester trying to make a movie so bad he could take over the world.