COIE AM vs. Emperor Joker, would be an extremely good battle imo.
On one hand, you have Joker, whom destroyed the whole of DC's post-crisis-creation, across all of it's actualities, with an ACME bomb the size of a tour bus (lol) - and who also warped/owned/replaced Spectre himself (along with every other character,) with the utmost of ease.
On the other hand, you have AM whom was able to warp the energies of the Central Battery sufficiently enough to own the Guardians with it (lol.) He destroyed all but 3 of an infinite amount of universes, then incorporated all of that power into his being. He also fought an amped Spectre to a standstill, and survived a creation destroying/rebooting blast (the same blast which coma-fied Spectre,) with very little ill-effect.
Personally, I'd lean slightly toward AM - though I don't see much they could really do to each other. *shrug*
AM also has the option to just chomping down the universe for more juice. It's the fact that he not only destroyed the multiverse, but that he did so before he absorbed all of it, that gives him the edge, IMO.
Then again, Mxy's powers let Joker basically say screw the rules, so it could be a stalemate.