Based on sheer size:
* The Death Egg: This massive space station from Sonic the Hedgehog 2 is arguably his most iconic creation.
* The Egg Carrier: Another giant space station, this one from Sonic Adventure 2, houses a powerful laser and is capable of interstellar travel.
Based on power and potential:
* The Metal Sonic: This robotic copy of Sonic is one of Eggman's most dangerous creations. He possesses incredible speed and strength, and has even managed to surpass Sonic in some instances.
* The Eggman Empire: Eggman has assembled a vast and powerful empire, equipped with a network of robots, weapons, and vehicles. This empire has caused untold chaos and destruction throughout the Sonic universe.
Based on impact on the Sonic universe:
* The Chaos Emeralds: While not a creation of Eggman himself, he often seeks to control these powerful gems, which have the potential to reshape reality.
Ultimately, which of Eggman's creations is "biggest" is up to interpretation. Each of these inventions has played a significant role in the Sonic universe and poses a major threat to the heroes.