I think that a lot of people above have interpreted the question as "What is your favourite game of all time?", which is quite a different question to the one being asked.
At the same time, im not really sure that I can come up with a satisfying answer, either.
If I consult metacritic, it tells me that the best game ever is The Legend of Zelda: Ocarina of Time, but I also have a hard time believing that gaming peaked in 1998, and nothing has surpassed it, since.
I also find it quite funny how - according to metacritic - 4 of the top 20 games of all time are just GTA V and its various ports.
Honestly, If I were to be completely objective, and think of what might be gaming's overall best achievement so far, I think I can sufficiently convince myself that it might actually be The Witcher 3, but im also certain that I can pick enough holes into why that game couldn't possibly be the best game ever.
"I don't know", is the answer.
Fuck it.
Peggle 2.