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.
Case Study of the IMPROMPER Way to Adapt a Challenging Character: Venom, Spider-man 3.
Case Study of the PROPER Way to Adapt a Challenging Character: Gorilla Grodd, "Grodd Lives", The Flash TV Series.