What makes a good game? There is no true right or wrong answer to that question, since it could be so many things.
It could be a huge commercial success, like Halo 3 or Assasin's Creed.
It could be a pioneer of it's genre, like Super Mario Bros. or Doom.
It could improve the quality of gameplay in it's genre, like Half-Life.
It could be very artistic and completely unique, like Okami.
It could be a missed gem that deserved more.
It could be known for it's mods, like Half-Life 2
So, what do you think makes a truly good game?