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