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.
Got all custom skills.
Got all collectibles.
Got all Red Star Rings.
Found the spring hidden in GREEN HILL Act 1 and reached the goal with a Red Star Ring.
Chatted with all of Sonic's friends you saved.
Cleared Sonic Generations.
Cleared all the missions.
Completed all of Sonic's friends' Challenges.
Got Rank S in all acts.
Executed a trick with a seven-step combo or longer.
Cleared half the missions.
Got Rank S in acts 12 times.
Performed all of Sonic's actions in Act 2.
Got Rank S in acts seven times.
Participated in a 30 Second Trial.
Joined the rankings.
Cleared CHEMICAL PLANT without entering the water.
Got Rank S in acts three times.
Defeated 100 enemies.
Cleared Green Hill within one minute.
Reached the goal without dropping any of the rings you collected in GREEN HILL Act 1.
Got Rank S in an act.
Completed the first stage.