I'm one of those people who loves one series and plays most of the characters from that series. That series for me being the Mario series and all characters relating, so Mario, Luigi, Bowser, Wario, DK, and Yoshi (Peach isn't very fun to me though, lol).
I'd honestly play, in some way, Waluigi, Bowser Jr., or even Toad if they were available in Smash 4, just because they fit in the series of the games I love.
Main-wise in PM though, I am stuck. I mained Doc in Melee and Wario in every version of Brawl I've ever played, but I really like that Mario is now back up to snuff. So I am now stuck playing both Mario and Wario in tournaments.