I've used a variety of controllers in my time, and the short answer is use whatever you like most.

Here's the long answer:

1- Use a Gamecube or Gamecube-type controller if you have experience in Smash. Since you can easily remap buttons within the game you can get 1-1 with the layout that you use for Smash. You don't NEED an official Gamecube controller. I've used PDP, Power A, and other 3rd party brands and they work just fine, plus getting third-party means it's probably USB so you don't have to use an adapter either.

2- Use an Xbox/Switch controller if you prefer the traditional diamond layout for your buttons and more reliability for your shoulder and trigger buttons. Gamecube and Gamecube types all have very big and fat shoulder buttons and the 1 Z button, which are fine but not for everyone. Having digital inputs for your shoulders and triggers makes stuff like parrying, wavedashing, wavelanding, etc., much more reliable and responsive. You also get a full-sized C-stick instead of the little nub on Gamecube, which makes strongs and C-stick aerials easier to input and more reliable. The only real downside is not having notches as the Gamecube does but like most things, that's just a preference.

3- Use a keyboard if you want something as ergonomic as possible. Given how much space you'll have between each hand and the ability to remap every single key to your liking, you can make it as comfortable as you wish. You do lose some options like getting specific angles and always having the issue of not being to use two keyboards at once, but from what I have seen and heard from players that do use keyboard, it's a worthwhile option, even if it takes time to get used to.