In fact Sonic started a slow decline after Sonic the Hedgehog 2 because the American and Japanese developing teams for Sonic split. They never fully got together (originally one group made Sonic 3(the Japanese team) the other group made Sonic Spinball (the American Group).
Wait what the hell?! Sonic the Hedgehog 3 & Knuckles wasn't good enough for you?
Bah.
I love and adore Sonic, as he is my favorite video game icon. I'm not a blind fanboy however, as I do have and control my moralities. Sonic games recently, save for Rush and Rush Adventure, haven't been of good quality. However, I think, Secret Rings, Rush, Rush Adventure, Sonic & Mario at the Olympic Games, Smash Bros. Brawl, and Sonic RPG all point to a good future for Sonic.