It would have to be Tails
I personally think that Knuckles would be better as a fighter than Tails, but based on the principles of Smash Bros., Tails should be first in line. Smash Bros., is supposed to be about putting famous video gaming icons into one big fighting game, I say supposed because that tradition was destroyed in Melee with the introduction of the likes of Marth & Roy, Pichu, Falco, perhaps the Ice Climbers, etc, and again in Brawl with the likes of R.O.B, Wolf, Lucas, Ike, and arguably Pit and Olimar. SSB64 almost got it right, but the inclusion of Ness messed it up slightly.
Anyway, if we're trying to stay on the lines of that tradition, then Tails should be in ahead of Knuckles, simply because he is more iconic, more famous, hes been around for longer, hes always been known as Sonic's sidekick.
Note: Going back to what I said before, I understand Nintendo had to scrape the bottom of the barrel to find characters to put in, as the number of famous video gaming icons is pretty low. Half the characters in Brawl would be gone if they stuck by their original plan.
But yeah anyway, Tails should be first in line.
But I dont think there should be anymore Sonic representatives anyway.
