9. Altered States
Director Ken Russell's 1980 adaptation of Paddy Chayefsky's novel is another mind-bending entry that might be deemed to stretch the boundaries of what we classify as science fiction.
William Hurt (in what was his first film role, remarkably) takes the lead as Dr Edward Jessup, a psychologist who becomes fascinated with exploring different states of consciousness, theorising that these can unlock the origins of human intelligence.
Through a combination of sensory deprivation techniques and a rare plant with hallucinogenic qualities, Ed starts experiencing these altered states for himself - but in so doing, begins to radically rewrite his own genetic structure.
As is the norm in a Ken Russell film, it's all trippy as hell and outrageously over-the-top, so it can be a little difficult to take seriously; even more so if you've seen the South Park episode that spoofs it.
Even so, Altered States delves into some fascinating subject matter, both as a speculation on the nature of consciousness and life itself, as well as a compelling drama about the destructive potential of obsession.