I have to go with Sonic 2 on the Genesis honestly. Sure it introduced the spin dash and allowed you to play as Tails for the first time* but I still think Sonic CD and Sonic 3K are much better. (Yes I know they came out later still I even like Sonic 1 a bit more than 2. :P) I felt that the game was a bit more broken than the other Genesis games, the music while still amazing isn't as memorable as other Genesis titles. Also it's disappointing that you can't fly as Tails, despite knowing he can fly. I also find Sonic Unleashed to be overhyped as well. I found Sonic to be a bit difficult to control at times, and the werehog while looking epic, his gameplay style just doesn't fit with Sonic himself... I honestly think the werehog's style of gameplay fits Knuckles more than Sonic himself. Maybe many people enjoy it since it was the game that came out after Sonic '06 and was "better than '06." (I like '06 more than Unleashed, sue me. :P)
*Tails didn't debut in Sonic 2 Genesis like many people think. He debut in Sonic 2 on the Master System and wasn't playable, I know most people here probably already know that but some people might not. xD