Sonic 1 Definitive is a hack named 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.