It's Sonic vs. Mario in the Craziest Smash Bros. Battle You'll Ever See
I used to think I was decent at Super Smash Bros. My friends and I would play all the time, and while I certainly wasn't world-class, I thought I could hold my own pretty well. But this video from ᴸᴼᴸᴵ ᴯᴿᴬᴵᵡᴱᴺ, which features Sonic the Hedgehog and Mario beating the ever-loving sh*t out of each other, takes things to a whole different level.
Kotaku pointed me to the video, and they say that this video was created using a mod for Smash Bros. Brawl called Project M. It appears that the mod makes epic battles possible that you wouldn't be able to pull off in the normal game itself, and from the look of this particular battle, that's definitely the case. Both players reach upwards of 500% damage, and there were numerous times I was certain that they'd both fallen off the edge of the level only for them both to come punching and kicking their way back up to the top again. It's a truly outstanding performance from both of them, and without giving anything away, I like how it ends. (Don't forget to stick around for Sudden Death.)