I'm on board with JJ here. He's right. It's like TV, sure the black and white box worked fine, but now we have slim HD TVs. Why would you go back? Because it's a thing of the past. It's not the same as when you first experienced it when it was new. You've already had better. Sonic Adventure was a great game according to the Dreamcast's standards. But I ain't playing Dreamcast anymore am I. If nothing ages, then why do we move on and continue to make things better? Because what we have is flawed compared to what we could have later. It's good right now because it's the best we got. It's like saying people are still as attractive as they were when they were younger. But then again, that's not the case for all games.
As for my favorite Sonic game it'll have to be between Sonic the Hedgehog 2, Sonic the Hedgehog 3/Knuckles, or Sonic Adventure 2 Battle. Colors and 4 were good but the others are just classics. I have yet to play Generations though.