Re: Marvels biggest slap in the face!!!
If we are talking pre-MCU, I'll go with the Captain America 1979 TV movie. If you've never seen it, you only need to take a glimpse at what Cap looked like to know how awful it was.
Within the MCU- SURPRISE!- Venom's my choice. The symbiote's origin was a cliche', his dialogue was completely out of character, and he was portrayed by an emaciated goofball.
Have a BIG BLUE Christmas and a Cat-tastic New Year.