Sonic Two for Mega Drive/Genesis is still awesome. Sonic and Knuckles is the best, with the whole backwards compatible feature.

Sonic Three for Master system is good in that you can sonic spin, and you can play as Tails. Sonic Two, on the other hand, drove me mad as I couldn't get all of the emeralds and there was some nonsense about hidden levels with a Silver Sonic and you could free Tails blah blah blah.

Therefore, for Megadrive I recommend Sonic Two, but for Master System/Game Gear I recommend the original Sonic the Hedgehog in all its glory.

Thank you.