Best and Worst Rock Collaborations?

We’re looking for the best in worst rock collaborations. The worst dynamic duos in rock. For example, we consider one of the best  to be Phil Collins and Phillip Bailey’s monumental rock epic "Easy Lover" and one of the worst to be R.E.M and KRS-1’s "Radio Song". Who got their musical chocolate in somebody’s equally musical peanut butter?

Please note, if your listed collaboration is a "best" or "worst".