My personal favorite title would be Sonic 1, with clever momentum-based platforming, with no SpinDash I might add- you have to earn that coveted speed, and you need it to make some tricky jumps, and I still like the visuals and music over Sonic 2. I also absolutely adore Sonic 3 & Knuckles for the music and visuals, and personally enjoyed most of the level design.
Regarding Sonic CD, I want to say it counts as you need the Genesis to even use the Mega-CD in the first place, but that's just me. If it did count, it'd be counted here as well for the cool time travel based levels and both soundtracks, though I'm sure we can all agree the bosses are a little too easy, aside from Metal Sonic. But hey, they are all really fun to battle, Just not Collision Chaos. The Pinball style design really wasn't enjoyable, but that's why we have the classic Sonic Spinball on Genesis, and that is probably the only Pinball game I personally enjoyed. (Sonic bias much. -_-)