Rovin I'd say Phantom of the Opera would be his most WTF movie I've seen, but this one is certainly full of some WTF on its own, particularly where the chimp pulls a perfectly fine and clean straight razor out of a trash can. But Argento's never let logic stand in the way of a stylish killin'.
Where else can you see an insane serial killing midget rip off his own face while under attack by a swarm of bees? Fully recommended.
Rovin Bonus nerd request: Needs Creepers tag, as that was its title in its initial (somewhat edited) North American release.
Goethe and ernie My favourite Argento film, for the soundtrack and the complete fucking nutsness.