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