On another thread a while back, I asked what Marvel villain simply can't be relied on to cooperate with others. That came out of an animated DC show I was watching where in the Joker was a member of a bad guy team, and I thought "No way. He'd crash the whole caper just to watch the pretty patterns of the shatter." In the same episode, however, I saw something else that didn't work for me: two master villains in a cabal that made me think "No way. If these two were each aware that the other existed, they'd probably be more focused on rubbing each other out than the good guys."
I'm curious about your take. Regardless of written history or varying character interpretations, what pair of villains do you think would be unable to tolerate one another's existence in the Marvel Universe?