Sonic the Hedgehog 2 not being there is crazy. The hype that game brought worldwide says they really didn't take any of their own criteria seriously. There was a reason the day of release was called Sonic 2sday.
An argument could be made for Sonic the Hedgehog 1 as it was the game that made the Megadrive/Genesis and thus Sega the success it was back in the day. But Sonic Adventure? Really? What historical impact did that have compared to literally creating the company's mascot and turning him into a world wide cultural phenomena?