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