Favorite would have to be my first Sonic game I've played: Sonic 2. Very fun, and I can't get enough of Emerald Hill's BGM.
In terms of the main series, my least favorite would have to be the very first Sonic game. I can't explain it, but to me, the original doesn't have the same "oomph" as Sonic 2 and 3 did. Not enough levels, and no spindash made me turn away from it.