Sonic CD is the best sonic the hedgehog game of all time. as much as the console/add-on it was made for stank like 3 week old cheeseburger, but out from manure grows beautiful flowers.
In this game, we are introduced to Amy rose/princess sally, who is kidnapped by metal sonic. so sonic has to rescue her, and destroy something known as the death egg. along the way, sonic picks up time stones which send him back in time, back to when the zone was primitive, and the future stones, which take him to the future, whether it be a good future, or a future in which Robotnik/Eggman rules the zone.
This game is considered by screwattack.com to be the best sonic game of all time, and rightfully so. according to them, "Sonic CD takes everything you like about sonic 1 and 2, and gave them upgrades." and later on states "It's just like super paper Mario, but 15 years Earlier!"
So trust me, if you have a gamecube, a sega CD, or a PC, get a hold of this game. you'll be thanking me when you're done.