Bird with the Crystal Plumage for me. Beautiful camera work and it's just got the most fantastic twist, although nothing comes close to the murky feeling he creates in Deep Red - like something totally alien on the loose - the scenes with Helga especially. Also Sleepless, Inferno, Four Flies. Opera looks fantastic. I enjoy it but it wasn't much of a stretch to guess the killer (same with Tenebre) - and then there's that ending. I've still got half of Phenomena to finishing watching
I wish he'd used Daria Nicolodi in bigger roles. She's gorgeous in Inferno. I was hoping she'd turn out to be the main protagonist.