I would definitely say classic Sonic. When the games depended on level design and interesting areas and puzzles to solve quickly. Not about having as many special moves as possible while attempting to gain even more. The classic levels really appealed to my love of exploring areas within a game, especially if I were playing as Knuckles or Tails. I could waste plenty of time just attempting to see what a random tunnel led to.
Modern Sonic games are mostly about surviving many many bottomless pits. There is not much to the level design anymore, at least not since Sonic Adventure 2 and that was a while ago. Now they are attempting too many gimmicks like Sonic riding vehicles, Shadow using guns, and Sonic turning into a large beast. The gameplay is no longer complex level design, in fact most of them are very straightforward, requiring the player to make very precision jump or time themselves just right to pass certain troublesome areas. Many times I find myself just making it through a level without much control over where I am going at all, just making sure I pass each jump or platform and land on every rail just right. The computer seems to do the rest for me. The number of enemies has dropped significantly, some levels only having just a few. Which are also taken out by the computer mostly through the homing attack.
I still think Sonic the Hedgehog peaked with Sonic 3 and Knuckles. It had the perfect mix of gameplay and challenge, while also adding a lot to replay value by being able to choose between Sonic, Tails, both, or Knuckles. The level design was so complex that you never had to go the same way twice, and each level was slightly different based on who you were playing at the time. Each character had their own unique ability while essentially playing the same. Modern games made playing as a different character far too different and the levels that require you to play as a character that you may not like is too much of a chore to get through sometimes. It actually makes me not want to play a few of the later games because of it.
And I generally think that none of the modern soundtracks could ever be as great as the ones from the classic games. Project Chaos has done an amazing job of remaking some of the early game soundtracks. None of the modern songs are nearly as memorable or well made in my opinion. This only causes the classic series to hold up even better when compared. In fact I don't think the modern soundtracks could be remixed in such a way and still be as good at all.
[video=youtube;-V10lnxggGQ]http://www.youtube.com/watch?v=-V10lnxggGQ&feature=player_embedded[/video]
Lately I have wondered if Sega had any plan for the character or series. That they believe as long as Sonic is in it the fans would buy it regardless of level of quality or amount of changes made to the game. I'm still a little hopeful for what this next game will bring, but until them I'm going to keep it to the 2D Sonic games.