Pro Controller.
I'm going to be honest with what I think. Most of the people who play with GC controllers just do it for the sake of it. Because they don't want to put effort into mastering a newer and better controller.
Pro is fine for smash.I am going to have to decide if I hate using the pro controller for Smash more than I hate dragging wires across my room
If you are planning to play competitively, GameCube Controller.
If not, just whatever you have at hand. The game was originally designed with the N64 controller in mind, ffs. You'll be fine. Just don't do single joycon.