Cronenberg's first feature film,
Shivers (a.k.a.
They Came from Within) defines his entire oeuvre immediately. It manages to simultaneously be exploitive, deeply horrific, and very intelligent, a perfect film for the mid-70's zeitgeist of sexual liberation and modernized living.
Out of print on home video since the 90's (used DVDs are currently selling on Amazon for over $100), so see it before it disappears.