Here is the full list of all 49 Sonic Generations Collection achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Got Rank S in acts three times.
Performed all of Sonic's actions in Act 2.
Executed a trick with a seven-step combo or longer.
Got Rank S in an act.
Defeated 100 enemies.
Completed the first stage.
Cleared Sonic Generations.
Cleared Green Hill within one minute.
Got Rank S in acts seven times.
Got Rank S in acts 12 times.
Got Rank S in all acts.
Reached the goal without dropping any of the rings you collected in GREEN HILL Act 1.
Took no damage while defeating the final boss.
Completed all of Sonic's friends' Challenges.
Chatted with all of Sonic's friends you saved.
Joined the rankings.
Participated in a 30 Second Trial.
Found the spring hidden in GREEN HILL Act 1 and reached the goal with a Red Star Ring.
Cleared CHEMICAL PLANT without entering the water.
Got all custom skills.
Got all collectibles.
Played Time Attack on all stages.
Got all Red Star Rings.
Cleared half the missions.
Cleared all the missions.