1. Mephiles the Dark. From the best sonic game; and sonic's most iconic rival. They could even make him really unique with chaos control too.
2. Pietro. Sonic's most iconic "helper" character. Pietro is one of the most fondly remembered one-offs in all of the Sonic fandom. His, "Who is the captain" quest brought me to tears.
3. I mean, who doesn't know Dr. Ivan "Eggman" Robotnik? One of the most iconic villains in the world, self explanatory.
4. Silver the Hedgehog. I mean it's no use escaping from one of Sonic's biggest frenemies, is there? One of the biggest requests in Smash history.
5. Elise. Elise is Sonic's equivalent of princess peach; only like a 100x cooler. Her romance with the Blue Blur is one of the most iconic in all of video gaems.
And finally... 6.
Na, na, na, na, na, na, na, na, na, na, na, na, na
6. Sonicman. Do you really need me to explain? He saved Elise, and fights bad guys! He is Sonicman!