I'm asking this out of boredom and raw curiosity, but I feel I will be opening a hives nest in doing so. Seeing as there are so many Sonic titles, quite frankly, what is canon? I know that sounds absurd for a platforming series, but I was frankly curious. Figure it would be fun to spend some time doing stupid shit that doesn't really matter, and pry the minds of anyone who might also like to think about said stupid shit.
Without further ado, this is my guess at the "main" Sonic series and the order I think they flow.
Sonic the Hedgehog
Sonic the Hedgehog Game Gear (Totally separate game from the Genesis title)
Sonic CD (Features no spin dash; a screen implies Tails is in the next game)
Sonic the Hedgehog 2 Game Gear (First debut of Tails; totally different from the Genesis game)
Sonic the Hedgehog 2
Sonic 3 & Knuckles
Sonic Chaos
Knuckles Chaotix (Spin-off but introduces the Chaotix)
Sonic the Hedgehog Triple Trouble
Tails Adventure (Spin-off)
Sonic 3D Blast
Sonic Blast
Sonic Adventure DX
Sonic Adventure 2: Battle
Sonic Advance
Sonic Advance 2
Sonic Heroes
Sonic Battle (Spin-off that leads into Sonic Advance 3)
Sonic Advance 3
Sonic Rush
Shadow the Hedgehog (Spin-off)
Sonic 2006
Sonic and the Secret Rings (Spin-off)
Sonic Rush Adventure
Sonic Unleashed
Sonic and the Black Knight (Spin-off)
Sonic the Hedgehog 4: Episode I
Sonic Colors
Sonic Generations
Sonic the Hedgehog 4: Episode II
Sonic Lost World
I am pretty sure I omitted none of the main titles, and the spin-offs I listed are perhaps the only 'reasonable' additions, as they're all platformers or made by acknowledged developers for Sega such as Aspect or Dimps. Rivals and Rivals 2, while also being platformers were never released in Japan, as well as Sega absolutely ignoring the Bioware RPG. The less said about Riders and their mobile-only titles, the better.