I have a very unpopular opinion most of you are not going to like.
First of all, I'm probably on a vocal minority of this, but I really don't like playing Smash Ultimate with an original GameCube controller. Why, well one, my hands get all slimy when using it for too long, even when I'm playing my GameCube with a game that's NOT Smash loaded. Lastly, the controller layout and design. The buttons are awkwardly arranged, with Y above A and X to the right of A, plus the buttons are loose when pushed down, which makes buffering a pain. This also plays a role in the trigger buttons. There's only 3 trigger buttons, which makes me claustrophobic with this controller. Also, the L and R buttons are analog buttons, and I'm not a fan of analog buttons when it comes to fighting games. Also, the Control Stick and C Stick feel weird when doing air attacks, especially since I set the back right trigger button as jump. It applies with how the controller feels in my hands. And the D-Pad is tiny AF.
Truth be told, I'm more comfortable with a Switch Pro Controller, or the PowerA GameCube Controller (the third party Pro Controller that is designed to look like a GameCube controller).
This is most likely due to how freakishly BIG my hands are. But I digress.