Troll 2 and the Room have both gone beyond being "so bad they're good" to arguably being masterpieces. I'm still somewhat convinced that the Room and Tommy Wiseau is actually a carefully crafted piece of performance art/social experiment.
Likewise both Plan 9 from Outerspace and Manos Hands of Fate are automatic entries onto such a list.
Pretty much all of the low budget monster movies that have come out in recent times... notably anything linked to the The Asylum or with "giant", "mega" or "shark" in the title... is an attempt to hit the "so bad it's good" market, although for me the most well known of these (Sharknado) was a little too self-aware for its own good; a truly "so bad it's good" movie thinks
isn't aware that it's bad and is convinced it's good.
Personally I think Humanoids of the Deep/Monster
deserves a place in this unlikely pantheon. For most of the film it's a fairly generic relatively low budget 1970's/early 80's monster movie/exploitation flick and so entertaining in a lowbrow way. Then, out of nowhere, you cut to scenes of the monsters raping people. And when I say out of nowhere I pretty much mean it; the rape scenes were filmed and added in during post-production by a backup director because the producer wanted them in there (notably they didn't tell any of the main actors). It's utterly bizzare and fairly hilarious.
The Wicker Man remake deserves a mention simply for the "not the bees!" dialogue.
I should note that I actually love cheesy 80's action adventures and schlocky cheap horror films so a number of the usual contenders for this sort of "award"... Commando, Road House, Hell Comes to Frogtown, Anaconda, Masters of the Universe etc etc... don't really cut for me as I don't think they're bad movies; they're in essence genre films which play entirely to what a genre audience wants. I think the same can be said for some of the worst blaxploitation flicks (the Black Gestapo being a notable one); they knew what a genre audience liked and pretty much just stuck to that; in that regard they're less bad films and more simply films with a very limited ambition.