Super Smash Bros. Brawl
Because mashing up just Nintendo franchises wasn't enough, Nintendo made a deal with the devils (or Sony and Sega, if you prefer) and jammed Sonic and Solid Snake in Super Smash Bros. Brawl. As the first non-Nintendo characters to appear in a Smash Bros. game, they caused quite a buzz, partly because we literally got drunk with anticipation.
They stuck out like sore thumbs, yet fit in perfectly. Snake had freaking grenades, and Sonic was a great deal faster than any other character in the game, making them unique and formidable. And it was an added bonus to finally see Sonic and Mario at one another's throats again after all that Olympic Games crap.