When I first started playing Super Smash Bros, I thought that it was about special moves only. I thought that punching was only a side thing. Oh, I was so wrong!
Mr. Game and Watch
I definitely mistook Mr. Game and Watch as a hard character to use (he still is, but not as much). His down smash can lead to some awesome combos, and most of his moves are similar to it. I had the same problem with Piranha Plant.
The plant's special moves are unique and are in a strange place between melee and ranged, which has 1 or 2 cons to it. Such as players being able to hit Piranha Plant freely. So when I advanced to the full moveset, I was able to beat my siblings easily in every match.
Pokemon trainer had a HUGE advantage with the full moveset. Ivysaur was way more powerful and was awesome with an open mind.
Fox, Sonic and other fast, flying fighters
Fox can be OP when you train with him a lot, and it's the same advice for Sonic, too. Here's a list of more fighters that are also lightning quick.
Ganondorf, Incinaroar and other slow, powerful fighters
Fighters that are the oppisite of fast and quick, like Ganondorf, are always really powerful and strong. Here is a list of powerful fighters.
So, in all, there are a lot of unique fighters, and you can learn the basic moves of all of them, or you can master one character. Either way is fun, and that's what Super Smash Bros. Ultimate exists for.