Very nice read, glad it was bumped up. I agree with pretty much... well, all of it.
Instead of offering my insight into the whole topic at hand, I figure I'll go through all of the Sonic games and sort of give them an ultra-mini review, as I know my view on them tend to be a lot different than most. All ratings out of 5. I'm not including the Master System games because I don't feel like it.
Sonic - 4.0 - The second Sonic game I've played. Awkward going back to not having a spindash, and the action being a tad slow - but this set the precedence for everything. Everything was just done so right for Sega's first attempt.
Sonic 2 - 4.5 - Sequels should expand, and that is exactly what this did. New character, new technique, better graphics, everything. This was also the first Sonic game I've played.
Sonic 3 - 3.5 - The level design was definitely better, but it didn't cater to speed quite as well. The ending was a little lackluster, and the whole package just didn't feel as good as Sonic 2.
Sonic and Knuckles - 4.0 - The best new character ever, fresh level design but keeping graphics nice and bright like Sonic 2. Expands a lot on the story yet not overcomplicating this simple platform with it.
Sonic 3 and Knuckles - 5.0 - It's just a lock-on, but it deserves it's own spot. The best hands down imo. It takes everything perfect from S3 and S3&K, and makes it even better. That lackluster ending from S3? lol that was only have of the adventure! Full save system, three different characters (first time Tails had his own kind of path going on really) with unique lines of play, and more.
Sonic 3D Blast - 3.5 - Very fun game with a new concept. The Saturn version is vastly better than the Genesis one.
Sonic Adv - 4.0 - Quality title making the jump to 3D. Going back and playing it later, it keeps a lot of elements from the 2D games and is the one 3D title that feels the closest to them. The Adventure field is nice, but gets kind of stale. Quality new characters.
Sonic Adv 2 - 4.5 - The best 3D Sonic title. Good that the field was taken out. Idk, everything about this game is so good, it's one of the best Sonic titles imo.
Sonic Heroes - 3.5 - There was a significant drop from Adv 2 to this, but this title still introduced a very cool mechanic that like no other game had ever done before. The ending was awesome. Started the unfortunate trend of levels being WAY too long.
Sonic and the Secret Rings - 3.0 - The game really just doesn't control that well which takes away a lot and docks some pointage, but besides that it's a quality game. It's very fun and the ending with Darkspine Sonic is one of my favorite endings yet.
Sonic The Hedgehog - 2.5 - People claim it's the worst Sonic game, but I disagree. There is still some fun to be found in this title. It's the worst of it's time, but come on - it's not that awful. The side missions are absolutely atrocious, but they can be avoided. Blaze on Emerald Coast (or whatever the beach level is) is legitimately fun. The levels in this game are beyond long, to where it's painful.
Sonic and the Black Knight - _ - I haven't played enough of this game yet to rate it. It's fun, but incredibly shallow. Doesn't feel like a serious game.
Sonic Unleashed - 1.5 - This is imo the worst title yet. I have only played the Wii version, so my score is based on that. It's seriously really bad. The day levels are practically uncontrolable, nothing makes sense, enemies are pointless, etc. It's practically holding right and pressing a button when the icon comes on screen. The same title could be made for the NES with a black screen where you have to hold right and press a button when it's shown on screen. The night levels are not a Sonic game.