I can list some of my favorites, but I wouldn't necessarily recommend them without knowing more about what kind of movies you like, because I don't think I'm a very typical viewer. I prefer narrative films; I gravitate toward horror-themed movies; I like bizarre and transgressive cinema; I'm straight but don't care if I see gay sex; and I watch these movies the same way I watch any other exploitation or genre movies (i.e., with a bowl of popcorn, not for their erotic utility).
A mentally stunted man with the ability to imitate voices naively kills several people before or after having sex with them. This movie isn't commercially available and I have only a poor-quality copy. I would love for someone to reissue it, but that doesn't seem very likely, because it contains rape, murder, necrophilia, explicit straight and gay sex, and implied incest.
If David Lynch made a porno... Scientists probe the sexual fantasies of a patient in a mental institution. I like this more than Cafe Flesh, which is another acclaimed movie made by the same people.
A long, dialogue-driven art/horror/porn film with bizarre themes and humor, written by cult director George Kuchar.
About a teen who's admitted to a mental institution and falls into the hands of a strange doctor (Fred Lincoln from Wes Craven's Last House on the Left) whose clinic is a weird S&M sex cult.
A damaged Vietnam vet embarks on a serial rape-and-murder spree. Has a lot of social commentary and some great performances. Up there with Water Power as the most notorious and controversial "roughie" of the 1970s.
I've been getting Vinegar Syndrome and Synapse's Blu-ray reissues of golden-age films as they come out, and my favorites among them are Thundercrack! (as previously mentioned), A Touch of Genie (a very Jewish comedy with a lot of non-sex performers), Throat: 12 Years After (a drama with some very caring and affectionate sex scenes), The Young Like It Hot (a comedy), and Sex World (which, if you view it as part of a trilogy with Westworld and Futureworld, is better than Futureworld in my opinion).