My favorite bad movie has got to be John Carpenter's
Big Trouble in Little China. Over-the-top acting by Kim Cattrall (when she was at least not unattractive) and Kurt Russell ("It's all in the reflexes"), a plot with holes so big you can drive Jack Burton's semi through it ("Who?... Jack Burton! Me!"). It has also spawned the only worthwhile bit of gangnam-style parody (YouTube "Lo-Pan style"... they even get James Hong to cameo in the parody!) there is... god, I hate that song!
I can't go a week without finding at least one quote from the movie that is appropriate and hilarious. So long as you go into the movie expecting terrible, you'll have a blast...
Big Trouble in Little China is one of my favorite corny/bad movies of all time.
As far as the worst of all time; I would have to list Ishtar, Battlefield Earth, Waterworld, American Beauty, Wall-E, The Contender, for starters...