For anyone looking to be competitive, I highly recommend the GameCube Controler, Pro Controler, or double Joycons.
I would not recommend the single Joycons or the Wii Remote with or without the Nunchuck because these just don’t have enough buttons for playing Smash at high levels.
A handheld Switch is the last thing I recommend personally. It feels weird on your hands spreading them out more than normal, and it feels weird looking down at the screen. Plus, it’s just going to make tournaments harder to run if that became a standard.
I’m sure that the 3DS and WiiU GamePad aren’t compatible with Smash Ultimate, but if they are, I wouldn’t recommend using them either.
As for me personally, I’m going to take my two Joycons and combine them into one controller. That works for me in most other Switch games, they’re still in good condition, and I have one of those holder things to make them feel like a real controller.