I voted for Phenomena. I love the way he shoots nature in the film. Very crisp and refreshing. I like Jennifer Corvino as a character. I think she's one of Argento's strongest heroines, and I like her defiant attitude mixed with her inquisitiveness. And that waterfall sequence is one of the greatest scenes he ever filmed.

That said, I also adore Tenebre. And Suspiria. And Opera. And Deep Red. It's so hard to choose!