Well, obviously, is the perfect game

. On a more serious note, there is no single perfect game. If you and your group can have fun playing is perfect while you play it. Sure, it may have its warts, but as long as you can walk away happy, what do they matter?
Moreover, the system used for a game comes secondary to its story. If there was a flawless system, a GM would still have to be capable in order to produce a perfect game. Some of the perfect games I've played include:
Star Wars: The Great Hyperspace Wars: The group had to fight Mandolorians, Sand People, and Sith, and we all loved the power that the characters had.
D&D: A drow-only game. All the treachery and deceit was a nice change of pace from our usual game. And I got to play a duskblade:cool:
D&D: An NPC-classed game. We had to wander through a castle with all its inhabitants killed by Billy, the 11-year-old psionic prince (it was freaky).
Exalted: The Circle got together and kicked some Dragon-blood butt. It was awesome.
You see, a perfect game can come from any system, as long as you play with the people you like and you can (at least for a little while) do the impossible.