I liked Sonic Adventure and Sonic Adventure 2, and I actually thought Heroes was okay. Heroes has the speed of the Sonic stages from SA1 and 2, level designs are fairly clever, and the soundtrack is great (the glitches and finicky camera kinda suck, but they're not really game breakers.)
Shadow The Hedgehog was the turning point for me: Stupid story, terrible "morality" and gun-toting mechanics, worse voice acting than the first 3 games, and bad glitches. The soundtrack again was good, but that wasn't enough to save it. After that, things went downhill with '06. Secret Rings and Black Knight did a small bit to bring the series back to being at least decent (Unleashed less so), but I just haven't had the same interest in the 3D Sonic games since Heroes.