if you want a hard, grating experience: Sonic 2 for Game Gear.
if you want a fairly easy one: Sonic 2 for Genesis.
if you want the overall best, Sonic 3 & Knuckles.
if you want a lot of platforming and some pretty awesome mechanics, Sonic CD.
if you want the one with the best soundtrack by far and heavily underrated for being different, Sonic 3D Blast for Saturn/PC.
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The future must protect the clone's past.
"Maybe it's a tentacle, molesting the planet itself. - Aschen Brodel.