Link for me. While I'm still not fully used to his new dash attack, it feels so great when it lands. Plus, he just plays much better than he did in Brawl making it even more fun to use him.
Villager is a very close second though. His moveset is a great tribute to Animal Crossing although I'll need more experimenting to really master it. I'm not sure I'll actually end up maining him though as it seems like there's a level of complexity to him with that will take more time to overcome than I'll be willing to invest.
I was a bit disappointed by Mega Man as his specials are just sort of mediocre. His smash attacks and some of his aerials are a lot of fun though, but I'm pretty sure he won't end up being as used as I was expecting.
I've only tried Pikachu and Mario once, but Pikachu felt pretty great. The person I played most in Melee and Brawl was a Pikachu main, so while I don't play Pikachu much, I know my way around its moveset. Thunder seems to behave a bit differently but otherwise Pikachu seems like it will be above average like it usually is. Mario seems pretty similar as well, but I just don't use his specials much since FLUDD is super situational, I tend to mistime the cape, up B is really easy to punish, and fireballs aren't too helpful in a two minute time match. As a result, I tend to fall into a rut with him, and I used him when I was still adjusting to the controls, so I didn't find him that fun. I'll give him another shot later, but of the demo 5, I'd rank him at the bottom. Once all the characters are available, he'll probably be closer to the middle of my rankings.