I've only seen The Fly, Videodrome, Naked Lunch, ExistenZ (which I own, oddly enough), and some of Scanners.
Haven't checked out Crash, Spider, Rabid, History of Violence, Shivers, The Brood, or any others.
OUt of the ones I'd seen, I'd say its a tie between The Fly and Videodrome. Naked Lunch was cool in a weird trippy way. ExistenZ had a cool idea to it too.