by Sonic Adventure 2 January 7th 2012, 3:01 pm
Great question.
My canon starts with the original games (Sonic 1, 2, 3, and Knuckles). These are the canon that started everything off. Spinoffs include relevant games such as Spinball, 3D Blast, etc. These are all stage 1 canon games.
Stage 2 canon games emerge with the Adventure series, Sonic Adventure 1 and 2. These are my only main canon games here. But this is where it gets tricky. I think there are secondary canon games which branch off SA2. These secondary canon games are ShadtH, Heroes, and Sonic 06. All three of these games continue from SA2, but no title officially claims the name "Sonic Adventure 3." Yet, any game featuring Shadow the Hedgehog after SA2 should be questionable for adding into the canon.
Then stage 3 came and ruined everything. There is no longer a stabilized plotline. We must suffer through games like Unleashed and Colors, where werehogs and wisps don't mix. The games that are "main" in this series are Unleashed and Colors, though I'm proud of neither, nor how they play. Branching off this series is a separate canon of storybook series, and all the other console games that don't live up to the first two canons.
Notice how the three stages differ in their terms of gameplay.
I think the portable games have a completely different canon than the console games, yet having the same aspect with stages. The two main stages here are the Advance stage, and the Rush stage. Everything else is a spinoff.
Sonic 4, Generations, and all other "reliving past-time" Sonic games fit nowhere in my canon, considering how they mix up timing within the series (or don't stay true to its linearity). Generations was okay, but the story was indecisive and really was a step back rather than a step forward. Sonic 4 just messed up everything. Generations could arguably be considered a spinoff, but Sonic 4 is neither spinoff or canon. These games belong to an imaginary canon.
I should outline this canon and post it somewhere...