I love Sonic CD's complexity and replay value, Sonic 3's length and scope and Sonic Mania's challange...
However, I have to choose Sonic 2 as the best preciselly because it is the opposite of the former three. It is so polished in its simplicity that, for me it is the closest to perfection a Sonic game has ever been.
I have to mention that the Master System version of the first game holds a very special place in my heart though... It is, by far, the Sonic game I've played the most.