As a person who likes Sonic games, I thought of Dark Gaia coming back but your not Sonic, you have to save him. You are Tails, Chip and Cheese. Metal Sonic is the enemy. It is a race to Dark Gaia's Emerald. More to come.
Metal Sonic's Game - Ultimate Flash Sonic (Online)
Why
I am not so keen on the newer 3D Sonic games. Personally, I think that running around getting rings is better than clawing Dark Gaia after the sun sets. There is a link to Free Sonic in the Sonic Sites section.
Big's Game - Sonic Heroes (PS2)
Characters
Here are the different teams for Sonic Heroes:
Team Sonic (Sonic Speed, Tails Flight, Knuckles Power)
Team Dark (Shadow Speed, Rouge Flight, Omega Power)
Team Rose (Amy Speed, Cream Flight, Big Power)
Team Chaotix (Espio Speed, Charmy Flight, Vector Power)
Chip's Game - Sonic Adventure DX (Gamecube)
Plot
Dr. Eggman learns of the legend surrounding Chaos. Believing it to be true, he seeks out the Master Emerald and shatters it, freeing Chaos in the process. Eggman's goal is to control Chaos, and use its destructive powers to collect the Chaos Emeralds and conquer the city of Station Square. To help him, he has created the E-Series robots, a group of robots powered by animals placed inside them and programmed to obey him. When Sonic learns of Eggman's plans, he and his friends spring in to action to stop Eggman and they all start their own separate, but intertwined, journeys for the Chaos Emeralds.