My two all-times favorites are definitely Tenebre and Opera. The Interwebz has lead me to believe that there's a significant number of people that think Martino rivals Argento in terms of who deserves the "Giallo King" moniker, but I've never been overly impressed with Martino's work (he's nowhere near as stylish as Argento, and he ruins otherwise excellent plots with very contrived and anticlimactic endings).
Besides Tenebre and Opera, my favorite gialli are the really sleazy and ultraviolent ones. I've heard Black Angel and The Monster of Florence are pretty over-the-top, but the sleaziest giallo that I've been able to get my hands on so far is Giallo a Venezia.