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