Majin Buu from Dragonball Z/GT takes on Wally West AKA The Flash from DC Comics(post-crisis, pre-NU52).
Fight takes place in the Grand Canyon, and both have prepped, with Majin Buu being high up in the air and ready to unleash a powerful attack, and Flash good and ready to run like he's never run before(which, for him, is saying a lot)on the ground.
Additionally, Buu is in his most powerful form, and the reason I put him in the air is that Flash would decimate him in seconds if they were on the ground together fighting. Anyway, who will win? The most powerful Dragonball Z/GT villain or The Fastest Man Alive?