Sonic 1: South Island Expedition

Sonic 1: South Island Definitive is a hack by RadiantNexus Team, which seeks to reimagine Sonic 1 by rearranging levels. remixing music, adding features from future games, and readding scrapped ideas in new and interesting ways.

Major features:

3 acts of reimagined level design, including one act that has yet to be fully reimagined in any other hack.
A miniboss.
Massive art overhaul all over the place.
Music made by ProjectFM and MechaKnuckles.
Menus!
A custom splash screen.
There's been massive engine overhauls as well, including the Sonic 3 level layout format for massive levels, and the Sonic 3 object manager to go alongside this.