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