Here is the full list of all 49 Sonic Generations Collection achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Played Time Attack on all stages.
Got all collectibles.
Got all custom skills.
Got all Red Star Rings.
Cleared all the missions.
Completed all of Sonic's friends' Challenges.
Cleared half the missions.
Took no damage while defeating the final boss.
Cleared Sonic Generations.
Chatted with all of Sonic's friends you saved.
Got Rank S in all acts.
Executed a trick with a seven-step combo or longer.
Got Rank S in acts 12 times.
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.
Performed all of Sonic's actions in Act 2.
Cleared CHEMICAL PLANT without entering the water.
Got Rank S in acts seven times.
Cleared Green Hill within one minute.
Reached the goal without dropping any of the rings you collected in GREEN HILL Act 1.
Defeated 100 enemies.
Got Rank S in acts three times.
Got Rank S in an act.
Completed the first stage.