Hands down the story arc of The Murder of Jean DeWolff was one of the most gripping in Spider-Man. He's threatened by all these supervillians with all these extraordinary powers and gadgets, but in the end, it's a domestic terrorist with a shotgun that deals him one of his biggest blows.
Sin-Eater has always been one of my favorite obscure Marvel characters. His weird color scheme almost seems to subvert just how dangerous the character is.

He started as an ML Machine Man with lower legs from Whirlwind; love that figure's bootcuffs