Realistically, its gotta be Brawl because it has pretty much everything the others have and more. The only things it lacks are most of the stages from the previous games (although a lot of them have new counterparts anyway), and very few characters, most of whom were just clones anyway - except Mewtwo. The only problem with Brawl is that they replaced Mewtwo with Lucario, but that's still kind of understandable anyway.