I've asked the question for Melee, now let's talk about Ultimate — which Super Smash Bros. Ultimate characters are viable for competitive play?

I know significantly less about Ultimate than I do about Melee, but I'm going to stick with my definition of viable. A viable character is a character that, when used by a top player, is capable of winning a major tournament without the player having to use any other characters for different matchups. Based on this definition, we know that Sora, Byleth, Roy, Pokemon Trainer, Joker, R.O.B., Pyra and Mythra, Cloud, Corrin, Wario, Steve, Fox, Sonic, Yoshi, Mr. Game & Watch, Bayonetta, Kazuya, and Pac-Man are all viable. Based on my research, it looks like Lucina, Min Min, and Sephiroth have all won tournaments as part of a player's arsenal, but never as solo characters. I am not sure about this though; like I said, I don't know nearly as much about Ultimate as I do about Melee.

What are your thoughts? What other characters are viable? Or do you have a different definition of viable that produces a different list?

I honestly think that the definition of viable is different from Melee to nowadays. In fact, I don't even think there exists such thing as viable.

Already back then, games had a lot of parameters that affected the performance of characters, cars or whatever was used in the game. But as years have passed, the games have evolved partly thanks to the more potent specifications of consoles to include a lot more mechanics, nerf characters in some franchises and in all including a ton more of parameters that affect performance.

It has come to a point where just because something is used by a pro player, or even the majority of players, doesn't automatically mean it's viable. What about your own playing style? What about the change in playstyle brought forth by the newly included and modified dynamics? That also affects how you perform at a certain game, and affects how good can you play at certain games of a long franchise.

If a pro player uses "something", then surely it has to be good because part of the success also comes from using that specific "something", which makes the strengths of that player's style flourish. But if you attempt to use the same "something" you simply won't use it the same way, and not necessarily because you're not as good at that specific game: you may have a different style which favors other parameters of the game and thus you might require other "something" in order to perform at a similar level.

I could continue on with this rant, but I feel like I'm heavily drifting off-topic so I'll simply say that I don't think there are viable characters in general terms. Ultimate has a ton of dynamics, battles are not the same as in previous games and characters have seen their performance modified. Thus, what makes a character viable for each player is that the character can merge perfectly with the player's style.

See for example how Jigglypuff was a main pro character in Melee yet apparently it has fallen into disuse even by characters that were already present in Melee. It's all because of the changes I've mentioned, and the fact there are so many characters that have won majors proves the many different styles the franchise can now favor.