For many people, it’s not Smash Bros. unless you can play it with a Gamecube controller- the Gamecube controller was what Melee, the second and most iconic entry in the franchise, was played with, and it’s become a mainstay of competitive Smash Bros. ever since.
The good news is, Nintendo is releasing Gamecube controllers to let you play the upcoming Super Smash Bros. Ultimate later this year- and those controllers will also coincide with the release of Gamecube Controller adaptors for the Nintendo Switch, which will let you plug those controllers in and play the game with them. These adaptors will cost $20, and will be available on December 4, which is when the game itself comes out.
Nintendo also made similar adaptors available for the Wii U (which, ironically, work with the Switch), but those were in incredibly short supply- hopefully, the ones made for the upcoming Switch games are not as impossibly difficult to find.