Tails or Knuckles?

[Rökkan]

That's it. He's never stood out as a character that could make it on his own, so why make him a fighter?

Sonic Adventure proves you otherwise.
Actually, the whole plot of the first Adventure for Tails was of him gaining his own independence from Sonic and growing as a character so that he could handle hero responsabilities on his own. He defeated Robotnik and saved the entire Station Square from being blown up.
In Sonic Adventure 2, that quest from his prequel reflects on him. He's much more mature and independent, saving Sonic from Prison Island and defeating Eggman (again).

The other games also reflects on this. Tails got more distant than Sonic in other games and less of a sidekick.

But even though Tails is my favorite character from Sonic the Hedgehog, I agree that playing as Knuckles would be more interesting. So I voted on Knuckles.

(even more interesting though would be to play as Robotnik!)