Going by the OP's criteria, I would definitely have to say Sonic & Knuckles for the good ol' Sega Genesis.
Yes, it's only half of a complete game (Sonic 3 & Knuckles); yes, it's missing features from the previous installment (sound test, 2-player mode, save function, etc.); and yes, Tails is nowhere to be found. Aside from all this, Sonic & Knuckles is extremely fun and a great game to boot.
You get to choose between two distinctive characters, Sonic or Knuckles, with distinctive abilities, you have the freedom to explore the Zones any way you please, uncover lots of secrets, including the Special Stages, collect Chaos Emeralds to go Super, have a boss at the end of ever Act, and has probably one of the best and most memorable secret final bosses in the entire franchise.
If that weren't enough, you can connect the previous 3 installments to Sonic & Knuckles to get and infinite Blue Spheres special stage, play as Knuckles in Sonic the Hedgehog 2, and you can play through both Sonic the Hedgehog 3 & Sonic & Knuckles while saving your progress.
Yes, it's only half of a complete game (Sonic 3 & Knuckles); yes, it's missing features from the previous installment (sound test, 2-player mode, save function, etc.); and yes, Tails is nowhere to be found. Aside from all this, Sonic & Knuckles is extremely fun and a great game to boot.
You get to choose between two distinctive characters, Sonic or Knuckles, with distinctive abilities, you have the freedom to explore the Zones any way you please, uncover lots of secrets, including the Special Stages, collect Chaos Emeralds to go Super, have a boss at the end of ever Act, and has probably one of the best and most memorable secret final bosses in the entire franchise.
If that weren't enough, you can connect the previous 3 installments to Sonic & Knuckles to get and infinite Blue Spheres special stage, play as Knuckles in Sonic the Hedgehog 2, and you can play through both Sonic the Hedgehog 3 & Sonic & Knuckles while saving your progress.