Suspiria is one of my favorites.
The colors are rich and weird - partially from the film stock - but also careful lighting, in the style of Mario Bava (who Argento worked with on Inferno).
Typical horror uses cobwebs and spooky castles... Argento defied that idea by using beautiful art deco architecture. Reminds me of the Karloff/Lugosi Black Cat, same idea - ultramodern, stylish and creepy.
The acting is spotty, but I find that to be a problem with a lot of genre Italian films. I read that Jessica Harper did her lines in English and the rest of the cast did them in Italian - since no one in Italy records dialogue on the set anyway - I can imagine that interfering with the acting.
Suspiria and Inferno were the first two in a trilogy that was never finished; Argento's finally working on #3. Details at the bottom of this:
http://www.darkdreams.org/trieste_report.html.
I heard that there will be a Hollywood remake of Suspiria. Starring Sarah Michelle Gellar, no doubt.
Leslie, what's your opinion?