The brainchild of Naoto Ohshima, the creator of the character of Sonic the Hedgehog.  This game was released for the short-lived Sega CD console. It is my favorite in the Sonic the Hegehog series because it features the game in it's purest form. Showing the power of the Sega CD, Sonic CD contained a kickass soundtrack, a altering story based on what you did in the past or future, and it also features the greatest Sonic special stage of them all.
The gameplay remains much the same as Sonic 2, however, by taking use of the Sega-CD's large amount of main memory (for the time anyway) the playing area is much larger and there are lots of intriguing new features. One of the more amusing features is the "shirinking" Sonic where one of Robotniks rays hits the heoric hedgehog shrinking him for an entire level. Although the game itself is relativly easy to complete, different game endings are on offer depending upon how you complete it. You need to complete the game with all 7 of the Time Chaos Emeralds in your possession which is quite a tricky thing to do. Fail to do so and the end sequence is cut short.
It seems strange that the person who has developed just about every Sonic title so far, Yuji Naka, had absolutly nothing to do with this title. It also seems that Sonic Team thenselves were not involved. Instead, this installment was created and directed by Sonic's designer, Naoto Ohshima. With this being the case, Sonic CD has a noticable contrast to all other Sonic games in both style and gameplay sporting a uniquely anime flair and a surreal fantasy story with psychadelic environments. It's possible that this is what Sonic was truely supposed to look like instead of the Yuji Naka ideal which is in all other Sonic games.
Yuji Naka and company's efforts were worthy in Sonic the Hedgehog 2 and 3.  However, it is the first Sonic and especially Sonic CD that really show off the best way a great, fast-paced platformer should be made.