When it comes to his directorial efforts, 'The Stendahl Syndrome', 'Opera' and 'Phenomenon' are very good. He also wrote 'Demons' 1&2 'The Crutch' (sometimes called 'Demons 3', but it's not a sequel) and he wrote the interesting but not always coherent 'Wax Mask' (directed by Fulci). His 'Masters of Horror' episodes are also good.

After "Tenebre" check out "Phenomena", "Opera" and "The Stendhal syndrome", all of them very good movies.
From Argento's 2000's works, i like "the card player", his Masters of Horror tv episodes "Pelts" and especially "Jenifer, and "Mother of tears" a flawed but very fun entertaining ending to the "Mater" trilogy.

If we're going to include the Demons films, I'd like to add The Church which is very underrated. Also not directed by him but he co-wrote it and produced it, and his daughter Asia stars in it.

every film he directed (except that comedy before Deep Red "Five Days In Milan" (I think it's called?)) between "The Bird With The Crystal Plumage" and "Two Evil Eyes" is excellent (in my opinion) including the 2 "Demons" films (which he didn't direct but wrote) "Trauma" and "The Stendahl Syndrome" are very good also.
One note though, while the "rock n roll" score of the "Demons" films worked very well I consider the use of it in "Phenomena" and "Opera" to be very curious I think it substantially repels the "eerie atmospheric tones" of those 2 films, especially "Phenomena"
still, "Phenomena" and "Opera" are very much worth seeing just the use of "rock n roll" is bothersome (to me)