What is the best Sonic the Hedgehog game

That would depend on your personal opinion.
Probably Sonic the Hedgehog or Sonic the Hedgehog 3! Sonic 2 is fun but sonic 1&3 are better! Sonic CD is really fun too. Knuckles Chaotix is fun too. So the best Sonic games are: Sonic the Hedgehog Sonic the Hedgehog 2 Sonic CD Sonic the Hedgehog 3 (my favorite!) Knuckles Chaotix Sonic 3 and Knuckles is the best.

Another opinion:
My personal favorite is Sonic Adventure 2, because of how much material it contains. There are two different stories to follow: the light and the dark one, and the action stages have five different challenges each, not to mention that there are six playable characters (out of which each pair are controlled the exact same way though). They really put an effort into the soundtrack of the game, and I still love hearing the old SA2 tracks. They have a lot of unlockable extra material like the Chao garden, a racing mini game and boss battles. The soundtrack of the game can also be listened to after unlocking it.
The Chao garden could be a game of its own cause it's just so cute and has many minigames and features.

Yet another opinion:
sonic colours



Next opinion: I believe that Sonic 3 is great, but Sonic 3 and Knuckles, a combination of 3 and Knuckles, is best.