Which villains (and heroes) would you choose in crossover?

Which Marvel heroes would you want to see fight against which DC villains and which DC heroes would fight against which Marvel villains? Do you think they'd handle it differently from the regular heroes?

How about Captain America and Spiderman take on the Joker?

@juliedc: YES! Joker squishes on the spider with his shoe, then proceeds to beat Captain America with a crowbar, signifying the downfall of America! Great idea.