Sonic 3 and Knuckles (the 2 games locked together to make the BIG game) has SO much to offer.
I have such a low tolerance now for sitting and playing a game (my younger self would be baffled by adult me). But you can still sit down and jam out a decent amount of the game quickly- and those early/mid levels are always fun to race through.
The game has a great pace (not meant to be a pun), amazing music, great level design, fun bosses (and mini bosses) and 3 distinct characters to run through the levels as/explore with. Also, Sonic 3 and Knuckles is the only game I can get all of the emeralds on (I still don't know the joy of being Super Sonic in Sonic 2... I can never see those bonus stage bombs coming!).
Not to mention, if you play any of the ROM hacks (Sonic 3 and Sally, or Sonic 3 and Amy Rose, etc), you get even MORE variability and replay value. Both games are SO well made. Sally works more like Mega Man/Samus (which is awesome in a sonic game) while Amy's hammer moves just add a lot of joy/replay value. Both characters are vulnerable when they jump, so their separate attack button just gives the games so much more variety.