Take Sonic and Tails, throw in Knuckles, and you have an awesome game designed for ALL THREE characters. Sonic alone is your typical Sonic game in which you have to save the day. Play as Tails and you can explore parts of the same game from different routes, also Super Tails is a badass. Play as Knuckles and you play along HIS routes which are separate from the first two and get a different side of the story.
Oh did I mention Tails can fly and swim? That means if you have a friend, you can do co-op, Tails will sadly get left behind a lot, but there are times where your friend can fly you around the levels. THE BEST old school sonic game, I got hours of fun from this, I still have it in it's original form also.