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