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Sonic the Ranking - list of platformer games in chronological order.

Sonic the Hedgehog (1991/16-bit)
Sonic the Hedgehog (1991/8-bit)
Sonic the Hedgehog 2 (16-bit)
Sonic the Hedgehog 2 (8-bit)
Sonic the Hedgehog CD
Sonic Chaos
Sonic Spinball
Sonic the Hedgehog 3 & Knuckles
Sonic Triple Trouble
Tails Adventure	
Knuckles' Chaotix
Sonic 3D (Blast/Flickies' Island)
Sonic Blast
Sonic Pocket Adventure
Sonic Adventure
Sonic Adventure 2
Sonic Advance
Sonic Advance 2
Sonic Heroes
Sonic Advance 3
Shadow the Hedgehog
Sonic Rush
Sonic the Hedgehog (2006)
Sonic and the Secret Rings
Sonic Rush Adventure
Sonic Unleashed (360/PS3)
Sonic Unleashed (Wii/PS2)
Sonic and the Black Knight

There are also Sonic Rivals and Rivals 2 but I don't count those in the main list because they're racing games. You may count them in your list if you feel like it, I guess.