This is a subject which I feel more strongly about than any other subject in gaming to be honest.
Back in the day, my sister had a Nintendo NES, which was technically "hers". My first system, recieved at the age of 4, was the Sega Genesis. So from a very young age, as much as I like the Mario series, I have always been a much bigger fan of Sonic.
On topic, the "classics" are better. Not necessarily just the fact that the 2D games are better than the 3D ones. Some of the crap that Sega has came out with over the past few years has pretty much made me sick of the Sonic series. I never, ever look forward to a new Sonic title being released, but rather go to look how degrating the series has gotten once I do hear about it. Sonic Rush was okay, but it wasn't as fun as the classics. Those games include Sonic 1, Sonic 2, Sonic 3, Sonic and Knuckles, and any of those that you locked on with S&K. Oh, and Sonic Spinball, but it was more side-gamish. Sonic 3D Blast was even a decent game. Sonic Adventure, whether it was part 1 or 2, or any variation in between, did not and will never match up to the magic or the caliber of greatness that was the classics I mentioned. They may have been good games to some, heck even great games to even less, but to me they will always be the first big mistake in Sega's attempt at recreating what was once awesome.
I find myself, almost on a weekly basis, telling someone somewhere how the Sonic series was so much better before. I own some of the best Gamecube games there are, including Wind Waker, Resident Evil (remake), Resident Evil 0, SSBM, etc., but I will always and forever stand by my honest word and say that Sonic Mega Collection is by far the best $40 I've spent towards my Gamecube.