Dressed to Kill (1980)

De Palma's always been a visual storyteller. See the museum scene with Angie Dickinson to get a practical understanding of what I mean - it's almost nine minutes of pure drama without any dialogue. It sets a precedent for a film that's essentially an American-made giallo infused with Hitchcockian style.
It's visually captivating, throwing split screen, split lens, amazing camera movements and deep focus techniques at the viewer in both striking and understated ways. If judged on style alone it'd get top marks without hesitation. But there are other considerations. Some of the cast, for example, were a little too inexperienced for the role they were asked to play.

4 spying eyes out of 5

NOTE: If you can't get a hold of the Unrated version, then you're probably better off not watching at all. The cuts made to the R rated version really weaken the experience. Don't even consider the TV version.