Only including films that have some kind of narrative (however thin), which excludes plotless gorenography like Slaughtered Vomit Dolls and made-as-a-prank films like Frozen Flesh, the top 10 worst feature-length (at least 60 minutes long) films that I've ever seen would probably have to be, in order:
1. Dancer in the Dark (2000)
2. Spring Breakers (2012)
3. Blood Sucking Freaks (1976)
4. August Underground's Mordum (2003)
5. Chaos (2005, David DeFalco)
6. Freddy Got Fingered (2001)
7. Pink Flamingos (1972)
8. O Fantasma (2000)
9. The Drawn Together Movie: The Movie! (2010)
10. An American Carol (2008)