For some reason, all of Argento's best films are currently available on youtube, and they look to be high-quality transfers. Ideally, you'd be seeing a movie like Suspiria as a pristine 35mm strike from the original technicolor negative, in a theater with stereo sound cranked way up and an enthusiastic audience. But this is the 21st century, and people don't go out to the movies anymore. Youtube will have to suffice. Here are my three favorites, in diminishing order:
You could expand this beyond a mere triple-feature if you wanted to; Deep Red, Tenebrae and Opera are also all on youtube in their original, uncut international versions. Everyone seems to like those. I suppose they're fine, but I was never a fan of his giallos. They don't speak to me the same way as these three surreal, maddening, insane horror films.
Bonus Feature: the 1985 documentary Dario Argento's World of Horror, an enlightening 70 minutes focusing largely on Phenomena (which I consider to be his last great movie). Some of the montages are too lengthy, making it drag in spots, but like Cronenberg, Argento makes for a good subject due to the insightful and articulate manner in which he's able to analyze his own work.
Related post: Zombi (Argento's Dawn of the Dead)