Ick, are you both serious?
Sonic CD is a complete and total waste. It has the speed of Sonic 1 and the level design of some kindergardener's drawing skills.
Sonic 2, while the 'hottest selling 16-bit title', still has a bunch of flaws as compared to Sonic 3. Sonic 2 had horribly designed special stages, reminiscent of what Chaotix tried to accomplish later on...
As far as I see it, Sonic 3 improved the formula on speed, bonus stages, and especially Super Sonic and Hyper Sonic (when attatched to Sonic and Knuckles, of course...) It saved your games, gave you the choice of 3 different power ups that did three different things, and had special level sets for Knuckles and Tails already built into the game. It also added a primary boss every time you finished a level. Different endings as you beat it with different characters... There's really no reason that Sonic 3 isn't the pinnacle of the franchise. You could go further and say SA:2, but I don't dig the 3D Sonic games; Sonic was meant to be in 2D as far as I'm concerned.
What would Sonic 4 have looked like? Damn, someone should make it.