Tusk is pretty high up there but that ones kinda so bad its good. 100% worth watching once and stay for the credits where it has a conversation from the producers hitting a bong and laughing about how ridiculous they can make the ending. The movie starts out totally serious but as it draws on it practically becomes a parody of itself.
The worst one I watched last year would probably be Mermaid Down. The plot make absolutely no sense in any way and the character development is a total joke. The 'bad guy' is just the cartoon level of 'evil because he is evil' and they try to mix in some strange ghost stuff that makes no sense. This is easily one of the worst movies I have ever seen and is not the 'so bad its good' kinda film.
The worst movie I have ever seen I think was called 'Gummy' but I can't even find it now. Some guy went across America and asked people for home videos in Burger King that he then scrambled into some very strange story of stuff he recorded. It would cut from kids killing cats to sell them to some guy to a guy whoring out his retarded sister to a methed out mom screaming at a 11 year old boy for taping silverware together to lift as dumbells. It kinda effectively drew out a very uncomfortable part of mentally unstable American society on lots of drugs. I think I was drunk off my ass and like 15 when I saw it but to this day that was probably the most uncomfortable I have ever been watching a movie. At least Requiem for a dream or Party Monster made sense and had a coherent story while this one was just insanity incarnate.