In term of characters, I want Tails or Shadow.
But in term of gameplay, I want Aiai since I'm curious about how his gameplay will be.
Will he stay in the ball all the time?
Will his super move got anything to do with banana?
Will he use his boxing gloves? Or a balance board instead?
I can imagine his Final Smash is literally giving whoevers playing him the power of tilting the entire stage how they please, thus they can try and shake and punt the opponents off the stage
with the stage itself.
My choice of which Sonic character to be in Smash would be Dr. Eggman. I can imagine him in the EGG-HVC-001 as his prime mech. It's modified to be more agile than it was in Palmtree Panic such as able to jump, punch and kick, but also fitted with a "quick-switch" function which would allow him to switch to other boss mechs from past Sonic games in order to fill his moveset.
Reason why I want Dr. Eggman for smash is because most of the fighters have had their villains invited to Smash already. Ridley, Sephiroth, King K. Rool and so on. And while I want Dr. Eggman as an original fighte,r it be the cherry on the cake if he took Metal Sonic along for the battle as Sonic's echo.