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.
EPILOGUE: The chimp ends up with a serious case of post traumatic stress disorder. He retires to a ranch in Texas. During a flashback he ends up ripping off the face of a woman and eating it.