Really? I've always felt the series hit its peak with S3&K. To me, it's a fantastic combination of speed, exploration, graphics, level design, music, and replayability. It's also huge, keeps the cast small yet diverse, and is quite a challenge without being cheap or stupidly drawn out.
I'm not knocking Sonic 2 mind you, as I too think it's a great entry. But it would rank second behind S3&K for me.