Sonic's got a few problems. One of these is adding so many characters, and the other problem is the plot, which often changes just for one game. Honestly, I can't count on my fingers and toes how many characters in Sonic Games only appeared a few time, but it's very rare that I want some of these characters to reappear.
Sonic games only are awesome if they're good, and unfortunately, after Yuji Naka left SEGA, I've been disappointed with Sonic's 3D stories. Sonic Unleashed did kinda well because half of the game was in 2D, but the game wasn't as good as it could've been. If they had made Werehog Sonic a bit less clunky, then the game would've fared much better.
The older 3D games, they've got such high ratings because they're good. Sonic Adventure and Sonic Adventure 2 are among the best video games I've played. If we get a Sonic Adventure 3 that's even close to the awesomeness of SA2, then I'd be happy.
Meanwhile, Shadow the Hedgehog can rot in hell, because as far as I'm concerned, the game never existed in the first place. A horrible concept with horrible gameplay. The only thing I liked was playing as Tails in two levels.
Not every Sonic title's a winner. The ones with High ratings are the ones you should get. The ones with low ratings, they got that way for a reason.