I might be over complementing it but, If there was only one Sonic game I could play, I would chose Sonic CD.
Sonic CD didn't stray far from the formula. The graphics were the same, not saying they are bad. But Sonic CD did add some more replay value and a great soundtrack. During each stage there would be a post. Sonic would be able to travel through time when he reached one. Times included past, present, a bad future, and a good future. Each one altered the level design. If I were to add all the levels up including the time alters, there would be over 60 stages!
There is also a new dash called the Strike Dash. Like the spin dash, it is a dash (obviously). Unlike it though, the strike dash takes less than a fraction of a second to charge up. But the bad part about it is that when strike dashing, you are unprotected. So you should look out for enemies.
Also now, there is a time attack mode. It allows player to replay levels for the best time. That includes special stages. There are other modes as well, such as the D.A. Garden (a specialized Sound Test) and the Visual Mode (contains various movies). Also, for the first time in the Sonic series, the player can save his progress throughout his journey! And yet, despite these great additions, I find Sonic CD's soundtrack makes me come back for even more. The game play still could of used more new features though and that is the main problem. Not enough change to the actual game play.
The game is the debut of Amy and Metal Sonic. That is probably the reason this game has become somewhat of a cult classic. I might be over complementing it but, If there was only one Sonic game I could play, I would chose Sonic CD.