Question: What is the greatest game, ever?
The answer, of course, is that there is no best spaghetti sauce.
That link will take you to 17 minute TED video with Malcolm Gladwell (and his hair) explaining how market researcher Howard R. Moskowitz realized that offering more variety allowed more people to be happier with the product they chose. It wasn't that one type of spaghetti sauce was better than any other, it was that people have differing preferences.
I think the same sort of argument can be made for games. Once you get past the basic mechanics of how the game is presented, it comes down to what the player wants. Simple or complex. Platform type (computer,box,tabletop,etc.). Solo or social. Stand alone or "collectible".
I could probably list a lot of other aspects of games, but I think you get the idea.
This post is inspired by a comment someone made to my November post about Netrek being the greatest Star Trek game ever. This might tell you something about how long I tend to sit on ideas, and that I am a terrible procrastinator.