In Sonic 3 & Knuckles, what is the name of the final zone?

The final zone in Sonic 3 & Knuckles is the Doomsday Zone.

The Doomsday Zone is the game's hidden or true final zone, following the Death Egg Zone. It takes place in outer space, where Sonic pursues Doctor Eggman's enormous Final Weapon while the planet and its future hang in the balance. Unlike the preceding stages, it is designed as a high-speed battle rather than a conventional platforming level.

Access normally requires Sonic to have collected all seven Chaos Emeralds, which unlocks his Super Sonic form. In the expanded Sonic 3 & Knuckles campaign, collecting all seven Super Emeralds also allows the player to enter as Hyper Sonic. The player races through space, collecting rings to maintain the transformation while avoiding asteroids and attacking the Final Weapon.

Death Egg is a common alternative answer because Death Egg Zone is the last regular zone and contains the main campaign's preceding boss sequence. However, Doomsday Zone is the final overall zone and the one associated with the game's true ending. The Final Zone belongs to Sonic the Hedgehog 2, not Sonic 3 & Knuckles.