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