I played AC 2 for a few hours and, I dunno, I just couldn't get into it. That's strange for me because I'm patient when it comes to games. Hell, I played through the first hour of Heavy Rain without giving up! But AC 2 always felt like it would never be able to suck me in.
Melee, meanwhile, is the most instantly accessible game ever. It's the game I can break out, regardless of who I'm hanging out with, whether it's my gamer friends, my family, or my girlfriend, and everyone will be up for a game. It's pretty much fun defined. Not to mention it's surprisingly deep when you look past the surface.
Melee gets my vote here.