Eggman is the best choice, if any. (With Knuckles out of the way)
Eggman brings personality, and he is clearly of a different build than Sonic and his pals.
Gadgets, robots, and other interesting shenanigans this villain could come up with does bring a lot of possibilities for Smash.
I do think that there is a great chance we will see Eggman as a boss fighter, which I'm ok with. His personality is brilliant, and would work well as a Boss for a story mode/Spirits thing. I would personally don't mind his inclusion as a playable either, but I think Boss is more likely.
Shadow could work as an echo. It's expected by others. (Thanks, Richter ...)
And, while I've been against it before, I'm "ok" with it now.
My first Sonic game was Adventure 2 Battle on the GameCube, and I fell in love with the series then, so Shadow to me is nostalgia, but ... The Shadow game kind of ruined the character for me. Now he's just filler.
I did not like the idea before, as to not wanting more characters from the same series, as 3rd party, even as echo, but now people have come to expect it, and I don't honestly care anymore.
I've always viewed Tails as annoying, but I respect his role in the series. (in terms of importance)
But, between Shadow, Tails and Eggman?
Dr. Eggman would have my full support. Perhaps now with all of this new views of Ultimate, perhaps I would want to campaign for him more for an eventual DLC spot. (unless we get a real surprise.)