Year released: 1991
Another obvious pick perhaps, but Sonic the Hedgehog gave credence to the bluster of Sega's much-hyped 'blast processing', a shorthand term providing much playground ammunition against Nintendo diehards.
Sonic the Hedgehog's gameplay was a compelling mix of tricky platforming and high-speed traversal, and it is the former that the newer 3D games still tend to struggle with. It also produced a memorable gaming villain in Dr. Robotnik (he's only called Eggman if you were born in the '90s, in my opinion), a totally hardcore scientist who enslaved cute bunnies and birds inside robot minions, because that's just how evil he is.
In hindsight, Sonic perhaps lacks the earnest timelessness of Mario, but he sums up Sega precisely—sky blues, high speed, and trash 'tude.