There are two types of characters that I prefer to use in Smash: Swordfighters, and Heavy hitters. My list of current and potential mains should reflect this.
Main 1:

I always have a blast playing with Marth and he has been my primary go to guy since his introduction in Melee (not counting the first months of Smash 4 before he was patched.) I've gotten really skilled at spacing thanks to Marth, and can more often than not land those satisfying tips.
Main 2:

I consider Ike to be tied for 1st place with Marth, but since I technically have more time put in for Marth (if only by a tiny bit) I've placed Ike at number two. Ike is one of my favourite video game protagonists, he's a common mercenary who's blunt, badass, and who doesn't give two damns about royalty; a very much needed breath of fresh air when it comes to Fire Emblem protagonists. He also falls under both my preference categories, so I instantly picked him up in Brawl. Been loving every second ever since, and I'm so happy he's finally getting the attention he deserves in Ultimate with all of his buffs.
Main 3:

Although not the best of characters, Ganondorf more than makes up for it in sheer entertainment. I have so much fun playing Ganondorf that he ended up being my third most played fighter in Smash 4. So happy to see him buffed and using a sword in Ultimate.
Main 4:

Link was my main in Smash 64, mainly because I'm a huge Zelda nerd, but I also liked his reach. Link is the sole reason why I like using swords in Smash and I've used him in every subsequent game since. Plus, Link himself is one of my favourites and BoTW Link's design is great.
Main 5:

Never played Punch Out!!, but I'm a big fan of boxing and liked the look of Mac's speed and power in his game trailer. His moveset just sort of clicked with me. It's a simple play style, but you also have to stay on your toes and be patient in order to be successful. My kind of fighter.
Main 6:

My first Melee main. I preferred Roy over Marth in my early days with the game because of his design, epic voice and fire attributes. (Little did I know at that time. XD ) Even once I switched over to Link and Marth, I'd always pull out Roy now and again for some fun. I was pretty sad when he didn't make it into Brawl and was really hyped when he finally came back in Smash 4. I didn't use him as much as my other 5 guys, but I still have fun using him time to time. In Ultimate though, he's looking like he's going to be pretty strong, so I'll definitely still be using him, and maybe even more now.
Since I have not yet played Ultimate as of this post, I'll list off the newcomers that I'll be trying out and may eventually come in somewhere on this list.

: Love him as a villain in Metroid, and he's got some interesting moves from what I've seen. I love high risk, high reward type characters, so I'll definitely be giving Ridley a try. So damn happy he's finally a fighter in Smash, given he's technically been in every Smash game. XD

: This was originally going to be Simon...until Richter showed up and stole his spotlight. XD Rondo of Blood is one of my favourite Castlevania games and I just prefer Richter's personality and design over Simon's. (Not to say that I dislike Simon the Barbarian, I still think he's cool, but Richter's cooler IMO) As seen with Marth, I like out spacing my opponents, so the Belmont's whip is right up my alley. His projectiles also seem really good and can cover many different situations.

: I love King K. Rool. The crazy eyes and expressions, the extra thicc belly, his wacky weapons and equipment, everything about him is what I like to see in a goofy villain archetype. I loved him in the DKC games and in 64 and ave been greatly missing him in the new DK games, so I'm really glad that Sakurai put him in. (Thank you, ballot) But even without my nostalgia towards the Kremling Kommander, his kit is looking really intriguing. He's heavy, but also mobile and decently fast and with a good recovery to boot. I'll for sure be putting in the hours labbing the heck out of him.

: The more I learned about Incineroar, the more I wanted him in the game. Beforehand, Decidueye was my pick for Gen 7, but come on, he's a macho Cat wrestler that's on fire; I just can't say no to that. XD People are giving him a hard time due to his slow movement and less than great recovery, but I main Ganondorf and Little Mac, so I'm used to bad recoveries. XD His power is looking mighty juicy though. Revenge and Alolan Whip look pretty busted, so I'm looking forward to using them. XD