As far as 'new' Sonic games go (as in this gen):
Secret Rings- Enjoyable. A bit of a learning curve, but once you get the hang of it it's a decent game with a lot of 'heart'. I'd give it a very solid 7.
Sonic 06- Meh. Some people have enjoyed it, but I find the controls, glitches, story, art and just... everything overbearing. 'Prolly would get a 4-6 by me, depending on my mood.
Sonic Unleashed (360)- FUN. Just... FUN. Great style to it, lots of hours of entertainment, a couple of glitches (Werehog/later game), but FUN. It gets a 7 or 8 in my book.
Sonic and the Black Knight- I find it hard to give it a rating. The music, fan work, story, and everything else it great. The gameplay... not so much. It has Secret Ring's camera system, but Sonic isn't on-rails. Once you get used to it it's not so bad (it kind of plays like a 3D NiGHTS), but the level design and controls just make it awful. Gameplay gets a 4-6 depending on my mood, everything else an 8.