Sonic Adventure 1/2 are some good shit, they have issues, but if they had ironed them out, they'd be some top notch games. The sonic stages are fun to replay, and adding the different goals in SA2 just adds even more fun to replaying the stages, even if they are a bit more linear then the firsts. They have their issues, but imo, the negatives out weight the positives in these two games. And having issues makes sense with them being the first real attempts at 3D sonic.
The best Sonic 3D sonic games, even if they aren't perfect, building off of the good things, and fixing the bad games would make for a great game. The story was top notch sonic stuff with over the top ridiculous world ending all the time stuff. Sonic story has always been over the top anime bullshit. Sonic literally went Super Saiyan in the second game.
Sonic Heroes was an improvement in a lot of areas, but a downgrade as well. The engine change made it somehow even buggier then before. The movement is less enjoyable as well. It had potential, but I think the negatives out weigh the positives starting here, and continuing in every game after. 3 games in they should've been able to fix the issues 3D sonic was having, but instead they made them worse.
Sonic The Hedgehog had some of the worst game development I've ever seen, and quite obviously suffered from it. I think it stays with all the other 'adventure' games, where it has a decent base, but it was never ironed out into a good game. There is some cool stuff if you can get past it being a inexcusably unfinished game.
Sonic Unleashed is the best boost game, and half the damn game is universally hated. I enjoyed it a little at release. I wouldn't mind seeing a boost game that builds off of this game, but I don't think boost is viable for the series in the long run.
Sonic Colors is mostly a 2d platformer, and all the 2d segments control like ass. I don't even think you can call it's 3d sections 3D. They are incredibly short and usually 3 lane sections where you strafe between the lanes. Basically on rails. The 3D movement is terrible at low speeds. The gameplay is downgraded completely from Unleashed, a terrible game. The start of the Saturday morning cartoon story bulllshit that has been Sonic stories for the last 10 years.
Sonic Generations is pretty much the same as Colors. If you enjoyed one, you'd probably enjoy the other. Just like Colors, it's mostly a 2D game, half the game is 2D retro sonic, that feels worse then the classic games. The other half is modern sonic, which is mostly 2D as well. Calling any of the boost games except unleased 3D is kinda a joke, any of the open areas are over in 5-10 seconds where it then goes back to being a '3D' lane, or more likely a 2D side scroller. They very much are 2D games, with short 3D segments.
I haven't played enough of any of the others to speak for them. Sonic Lost Word, and the Sonic Boom game are at least somewhat interesting for trying something new.