Well, I judge a game on several different things, all of which change depending on context. For instance, I judge relatively linear RPGs (mainly j-RPGs) on story and character development. Graphics normally don't bother me that much if the story is good and the characters original. My perfect 10 for this would have to be Chrono Cross, since there is such a huge array of characters to choose from and an incredibly well designed storyline as well.
However, for other games, I generally judge them on how well they play, customization (per player), how fun they are, and how intuitive the game plays. IF it is easy to learn and implement and I can make my own characters and choices, I generally deem it a good game. MY perfect 10 for this area would have to be the PC version of Oblivion, due to the shear amount of customization that can be put forth into the game. I would also have to list Fable II in here, becasue I found it profoundly amusing to play.
My last category is really simple, just your average game that says "I'm entertaining and you don't have to do much." Platform games, I think they're called, like Spyro the Dragon and Jak and Daxter. They are fun, relatively mindless, and easy to play, made purely for the entertainment factor. Incidentally, those 2 would be my perfect 10 for this category.
Sorry it got so long-winded, but there you go.