I won't say what's the best fighting game but imo what sets Smash apart is the set up in it, you have a load of mechanics that the player is left up to their own ability to utilize in a sand box fashion. This leads to a metagame that constantly rotates in each of them where the is no set approach to playing the game.
In other fighting games you have an overall system that all characters bend to it's design while Smash is the opposite where you pick the character and bend the system to how you want to play due to the sandbox like approach to the mechanics. Just yesterday in another thread we're looking at the Bidou tech that is now being explored in S4 and people are finding crazy uses for it, this is much like in Melee when people began realizing the potential of the mechanics.
Before I was skeptical on Melee's longevity but looking deeper at the series with the type of metagame the Smash games have and the sandbox like design to fighting I can see these games being competitively played in big tournaments for decades.







