Some of these answers sound like they are proposing the most disgusting gag versus the most humiliating one, but I guess maybe for some the two would be one and the same. For me, gross versus humiliating are different things, so I would so penis gag is most humiliating. But also Heternno said "There are models that replace the usual mouth piece with a rubber cunt, or even a rearhole. Your face just suddenly became a hole for use," and I agree that sounds pretty humiliating also.