Sonic 2 added the Spin Dash — solving the original's standing start problem — introduced Tails as a co-op companion, and delivered longer levels with more complex route branching.
Follows: Sonic the Hedgehog
The Spin Dash sounds like a minor addition. In practice it transformed the feel of playing Sonic. The original required running space to build momentum — a constraint that shaped every level in the game. The Spin Dash freed level designers from that constraint. Characters could begin a section at full speed, could restart after a hit without needing a runway, and the game felt more fluid as a result. The move became so fundamental to the Sonic experience that the series has included it in almost every subsequent entry.