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