I happen to be a pretty big fan of cult films and there are several here I haven't seen. Great question! Don't forget
Attack of the Killer Tomatoes
Peculiarities of the National Fishing
Motel Hell(o)
Soylent Green
Eraserhead
A Boy and His Dog
2001 A Space Odyssey
Brazil
Run, Lola, Run
Buckaroo Bonzai - Across the 8th Dimension
My favorite - Doctor Strangelove
For gore, it's hard to beat House of 1000 Corpses
If you haven't seen Local Hero, it's a great movie but very hard to find. Mark Knopfler (Dire Straits) did the soundtrack.