Board Thread:General Discussion/@comment-25819077-20150418230622/@comment-4533965-20150419205928

Beren Camlost wrote: Which games constitute the mainline of the series (big ticket console games). Some, like SA1, the original, and Unleashed, are undeniably members of the mainline. Others, however, such as Shadow, Chaotix, and 3D Blast, are much more debatable. In order:
 * Sonic the Hedgehog (1991)
 * Sonic the Hedgehog 2
 * Sonic the Hedgehog CD (Sonic 2 and Sonic CD have an odd placement, since some fans tend to believe CD is before Sonic 2)
 * Sonic the Hedgehog 3 & Knuckles
 * Sonic Adventure
 * Sonic Adventure 2
 * Sonic Heroes
 * Shadow the Hedgehog
 * Sonic the Hedgehog (2006) (it's unknown if it's still in-canon, since the events were erased but it was acknowledged in Sonic Generations)
 * Sonic Unleashed
 * Sonic the Hedgehog 4: Episode I
 * Sonic Colors
 * Sonic Generations
 * Sonic the Hedgehog 4: Episode II
 * Sonic Lost World

Some like to argue that Sonic Rush and Sonic Rush Adventure also count in the main series, but they're considered in the handheld series. The Sonic Storybook Series also is considered sub-main series due to the fact that the events of Sonic and the Secret Rings was briefly mentioned in Sonic Generations.