Well, as for my opinion...
Old Sonic games: I laugh at the fact that people keep saying this was the era where Sonic was "fast". Try standing still in front of a hill and try to walk up. Fast my ***, Mario could keep up with that. The only way Sonic could go fast was by going downhill or charging a spindash that would take forever to charge and would last for half a second either because you would go up a hill again or would bump against a rock. Trying to jump over it murderd your speed too. Beside the horrible slopes and bad visibility, it was rather decent.
Sonic Adventure series: For me, these games were Sonic's best. Speedy games IF you could keep up and knew how to do a decently timed spindash while running, and it had a decent story. A big downside was the fact that some character's levels sucked ***. Big, Tails/Eggman in machine, E-102, Amy, and Knuckles/Rouge. Basicly, I only like Sonic, Shadow, Tails, and Knuckles, so it wasn't a big deal for me, but it might be horrible for players who love characters like Knuckles alot.
Sonic Heroes: Nice and fast, and not that bad when you think gameplay wise. It does have some horrible story line issues, and team chaotix was irritating. Its also not that exciting as a game. I hate it that team switching had to be forced, though. Or that you had to take a team with you in the first place. I would always let my teammates get caught by those robots and run further as Sonic alone.
Shadow the hedgehog: I expected a Sonic-like game. I got cool wanabe crap instead.
Sonic next gen: I feel like it could have been a nice game. If you would you just concentrate less on the enemy fighting, more on the speed, take more time on perfecting the engine and adding multiple paths, and seriously improving the story cutscenes, it would be nice. It doesn't feel like a bad game at all. It feels like it could have been way better. It seems rushed.
SatSR: Nice game for a while. Just lacks real levels. I would be happier if they threw the missions away and made more levels instead, with only the nice "Get To The Goal" missions and boss missions.
Sonic Advance series: Just like the old Sonic games, but a little better with the engine and graphics, and a little worse level design.
Sonic Rush series: All the good of the newer Sonic games, like homing attack and speed, with the good of the older Sonic games. Its just too bad its so short.