My main is Ike, and he has been since Smash for Wii U & 3DS. I didn't really have a main in N64, Melee, or Brawl, I'd just use whoever. He's from my favorite video game in general, Fire Emblem: Path of Radiance, and I really like the way his gameplay in Smash Bros. feels compared to the rest of the Fire Emblem characters. I'm comfortable with his slower speed and heavier weight in exchange for his more powerful swings.
My secondary somehow became Samus in this game. I've just really found myself having a blast using her and find her to be really effective for me. She might have a case to be my main since she might be more reliable for me, but the combination of my affection for Ike's games and his gameplay in Smash just gelling well with me for whatever reason leaves Samus as the trusty alternate.
Lucina's probably third. I would rather be using Marth since I don't enjoy Awakening, but Lucina is just the better Marth in Smash Bros. Ultimate, so it makes more sense to use her instead.
I like using a lot of the roster for fun, but the other best ones for me are Bowser, Donkey Kong, Hero, Lucas, Robin, and Roy.