I believe whole heartedly that Argento perfected the Giallo formula with Tenebrae. Why? Well, much of the time Argento's giallos don't exactly work as pieces of narrative cinema, and I think you could make a case that he's a superior visual artist or even art director than he is as a director. Suspiria, Phenomena, Deep Red, etc.. Beautiful, sumptuously colorful and violent images but I honestly forget the plot as soon as I'm done watching them. Suspiria isn't even truly a giallo, given that Giallo films differ from horror by always shunning supernatural elements for crime, mystery and investigations. Tenebrae, however, HITS. It makes use of a simple whodunit structure with a nice if easy to foresee end of film twist but manages to imbue the simple narrative with much thematic contemplation on wounded masculinity, misogynistic rage, and buried memories. A visionary film.
Also love Bava's Blood and Black Lace, Aldo Lado's Short Nights of Glass Dolls, and Fulci's a Lizard in Woman's Skin