Here's my theory on the five types of Sonic game. Instead of lumping the entire checkered legacy of Sonic together, it helps me to divide them into types like this to better rationalise and understand every direction Sega have taken their mascot in. This is just my perspective, feel free to sling mud or whatever.
TYPE 1: Retro 2D Sonic games. The originals. The classics. The ones we all love.
- Sonic the Hedgehog, Sonic CD, Sonic 2, Sonic 3, Sonic and Knuckles, Sonic 1 (GG/MS), Sonic 2 (GG/MS), Sonic Chaos and Sonic Triple Trouble. Knuckles Chaotix is the divisive one that may or may not go here depending on your opinion, but probably not. It's design could put Chaotix as Sega's earliest crack at TYPE 2.
TYPE 2: Modernised 2D Sonic games. They're 2D, yep, but they couldn't be more different. Inspired by the style of the 3D games, they include the same sort of attitude and art style, but usually with better results due to Sonic just plain being BETTER in 2D, even if it's modernised. Sonic speeds through rollercoaster levels with little obstacles/platforming, often doing tricks and grinds. Unleashed and Colours sparingly use 3D in levels, betraying the 2D but retaining the gameplay ethic.
Unleashed is the real oddball here, as it has hub levels, Werehog levels, bosses and Tornado levels in addition to the Sonic Day levels, but essentially it could be described as being Sonic Rush with just more gameplay types and a much bigger scale.
- Sonic Advance 2, Sonic Advance 3, Sonic Rush, Sonic Rush Adventure, Sonic Unleashed, Sonic Colours, Sonic Colours (DS).
TYPE 3: Those caught in the middle of Type 1 and 2. The middle children. Neither completely retro nor completely modernised, they serve both equally. Usually retro-style in basic gameplay or looks, but endowed with a hint of modernisation in the art style or the design that keeps them from being considered as truly 'just like the oldies'. Two pretty lovely games that bring both sensibilities together, no matter how much Sega wants you to believe they are TYPE 1.
- Sonic Advance, Sonic 4: Episode 1.
TYPE 4: The failed 3D Sonics. Sega's constant attempts foiled by bad design, unfinished development, terrible understanding of their demographic, ridiculous gimmicks, RAWK music, multiple Sonic friends, bats with tits and the obsessive intent to turn Sonic into something he's not. A promising start to these games led quickly to a very steep decline in quality.
- Sonic Adventure, Sonic Adventure 2, Sonic Heroes, Shadow the Hedgehog, Sonic and the Oh God no please make it stop! 2006, Sonic and the Secret Rings, Sonic and the Black Knight.
TYPE 5: The spin-offs. The minigames. An eclectic collection starting with the humble Sonic Spinball stretching all the way to the recent All-Stars Racing, this is basically everything else.
- Sonic Spinball, Robotnik's MBM, Sonic 3D, Sonic R, Sonic Jam, Sonic Shuffle, Mario and Sonic both games, All-Stars Racing, Sega Superstars (Eyetoy), Superstars Tennis, Sonic Riders, Zero Gravity and Free Riders.
Looking at this template helped me realised that TYPE 1 may never be attained or added upon ever again. Type 2 is something we can accept as an alternative if you like 'em enough, but it's down to the individual. TYPE 3 are rare breeds and simply the best Sonic games we'll play in this day and age, mainly because Sega aren't the same company they were in the past and this is (seemingly for now) the best they can do.
In conclusion, personally I'm really looking forward to this Wednesday (I'm in the UK) purely because if TYPE 1 can truly no longer be attained, I'd rather have a TYPE 3 game any day of the week than the alternatives. Bring on Sonic the Hedgehog 4. Let it stand on it's own merits rather than be judged on all this horrible stigma it doesn't really rightly deserve.