The classic. This game was a complete evolution of Smash 64, with so many improvements and good additions added to the formula. Adventure Mode, Board the Platform, Homerun Contest, Event Mode, and all the trophies to unlock. That's not even getting into the gameplay improvements and the engine that allowed all the sick movement options and tricks used today.
This is the Smash game I played the least. It is very good, but its appeal to advanced players don't get me much, as a casual player. Its graphics and style are pretty good, and have many single players options that are pretty good. What hurts it for me is the have physics and how fast it is, exactly what makes it good for the majority of the people who prefer it over the other games.