What would I add? It depends if you mean "cult" or "underrated" as I think there's a difference and this list straddles that line in a few cases. For example, The Rocky Horror Picture Show is unquestionably cult, whereas I'd argue The Man Who Would Be King is merely massively underrated and underseen.

However, for "cult", I'd add things like Shaft, Psychomania, Ride with the Devil, El Topo, Dark Star, Wake in Fright, Switchblade Sisters, Performance, Harold & Maude, Deep Red, THX 1138, Tommy, Celine and Julie Go Boating, Pirhana, Theatre of Blood, Beyond the Valley of the Dolls, Enter The Dragon, Eraserhead, Two-Lane Blacktop, The Cars That Ate Paris, and possibly Mad Max and A Clockwork Orange, though I think those last two are now rather considered straightforward classics rather than cult films.