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In all likely hood i'm the boards biggest Argento fan, and like most everything he made. But since you're new to Dario's films, i'll recommend a few that i'm 99% sure you'll love.
Inferno (very Gothic, plays out like a dark dream, sequel to Suspiria) one of my favs
Phenomena (gory, supernatural, and Giallo elements, and Heavy metal)
Deep Red (make sure you see the unedited version, that's over 2 hours long)
Sleepless aka Non So Honno (very graphic modern day Giallo)
The Bird with the Crystal Plumage
The Stendhal Syndrome (a wicked nightmare of a film, very surreal)
Suspiria
Opera
The Mother of Tears (2nd sequal to Suspiria, a Gory masterpiece, probably his most graphic film)