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