Just read this article about the type of controllers supported with the game.  Your options are:

- Gamecube controller (released by the company and via Wii U GC adapter)
- Switch Pro Controller
- Joy-con (set)
- Joy-con (single using horizontally)

Personally I love the fact that you can use a single Joy-con and that's how I will play.  I don't intend on having more than 2 sets of Joy-cons so I can have up to 4 players, and since Nintendo has always been my party system this is a big deal for me.

So how will you play Smash Bros Ultimate?  

https://nintendosoup.com/here-are-the-controllers-supported-in-super-smash-bros-ultimate/

Well, I'll be using the best Nintendo made controller in history.

So clearly, I'll be using the Switch Pro :P

Though I will do a lot of handheld play which is fine, I like that setup too. Had no problem playing Doom handheld.

Probably the Switch Pro Controller, i absolutely love that controller

I voted Switch Pro but of course when I play in handheld I'll be using the Joy-Cons.

Probably joycon set.

Don't yet have the pro controller, otherwise would most likely use that. But I do like using default controllers for systems, so that if I'm ever at someone else's place and that is all they have to use, I don't feel handicapped, ect.

Always annoys me when I play someone in a competitive game and after beating them they resort to. "I'm not used to this controller"

Also a reason I HATE QTE events. I hate seeing some Y, X, triangle, A, B whatever on the freaking screen, cause none of the controllers use the same location or thing. If you only ever use one controller in your life, then it's fine, but for those of us who use multiple due to multiple systems, its confusing as heck. I can swap controller easy enough, cause button locatiosn the same, but the button names aren't so QTE's beyond a shitty gameplay mechanic just confuse me.

Gamecube is the only way for me. If anyone is planning on using Joycons competitively, I don't recommend it based off of playing Smash 4 on PC with CEMU. It's not very comfortable. Think I'm gonna stick with the Gamecube controller released for Smash 4.

Yea itll be a challenge to play with a single joycon...maybe relive the days when I played Street Fighter on SNES controller (but tinier lol)

I agree that the gamecube controller is a bit overated as the better controller for Smash but I guess some people love using it, possibly due to the nostalgia and that those people played Smash heavily on the Gamecube.  Personally I think Nintendo's Wii U pro and Switch pro controllers are far more comfortable for me to use to play Smash.

Switch PRO-Controller for me.

Gamecube controller, cause I'm too much of a Smash purist.

Gamecube controller since i want that dose of added nostalgia besides seeing everyone in the roster

if not definitely joycons then Switch Pro controller after i get my Switch in late June to July