Super Sonic Sequel!
Sonic the Hedgehog had a lot going for it, but Number 2 improved on all of the features making for more of an exploration-heavy challenge, and includes a lovable new character! Sonic 2 is easily one of the best games ever made.
The levels are very large. The amount of paths that can be taken include many lampposts that can be used as special stage portals, meaning there's more reason to try and find them! Plus, the music is so much better, bringing in a lot of new and unique tunes that match there levels perfectly. Mystic Cave is a personal favorite. And then we have the new fun and colorful Special Stages, which offer a great challenge that helps emphasize Sonic's speed, making it very suitable. And finally, we have Tails, a lovable fuzzy sidekick that can act, believe it or not, as a second playable character, even in 1 Player Mode.
Sonic the Hedgehog 2 added a lot of great features to the franchise, including stage themes, special stages, and characters. It's easily the best Sonic game ever made.