I enjoy playing as a LOT of fighters, lol. I could have even included Bowser, Dedede, Pikachu, Sonic, Mewtwo and Bowser Jr. here too...
Basically, Smashdown was made for me.
Anyways:
Richter: The most interesting/fun-looking of the newcomers to me from a gameplay perspective, and Castlevania 1 has been one of my top 3 favorite NES games for years. Looking forward to making him my new main, and his white+pink look is fucking fabulous. Edit- I know Simon is from Castlevania 1 and Richter is from Rondo of Blood/Dracula X, but while Castlevania 1 got me into the series, I think Richter looks better in this game.
Marth: Never played his archetype before but always wanted to. Apparently Marth is really good in Ultimate which unfortunately does sway my opinion slightly.
Villager: My Smash 4 main. I may switch between him and Isabelle depending on how much I like her changes, but in general Villager was the Smash addition I always wanted (and they even kept my preferred look for him, instead of the lankier New Leaf "people.") Moveset is fun and interesting and he has some of the best taunts in the game.
Lucas: The Smash 4 version of Marth for me; didn't play him or Ness much, but wanted to learn, and I'm a fan of Earthbound Zero and still pray for a US release of Mother 3. Lucas is also the only Amiibo I own.
Luigi: My 64/Melee/Brawl main. Didn't like some of the changes in Smash 4 and I don't like some of the changes I see in Ultimate either, but when I play him it's like riding a bike. He's a very standard, fundamental fighter, and I know his moveset like the back of my hand, so playing him is always really natural.
Toon Link: The funnest Link. I was overjoyed when Young Link was replaced by his far superior Wind Waker descendant (or whatever). Unfortunately ever since his first appearance in Brawl he's become less and less "Toon" which is lame but whatever.
Piranha Plant: Looks like a lot of fun (and much more up my alley than past "weird" chars G&W, ROB, and Duck Hunt) but I'll make a final call when he's actually released.