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