As we can see, asking "What Is A Good System" on a board such as this is almost akin to a religious debate, with each liking and disliking a number of different systems and settings.
That being said, for all that I prefer some systems over others, I find in the end it is the interaction between the GM and the players that makes or breaks a game.
While I am not overly fond of GURPS, I was in a fantastic GURPS Tekumel game, due to good GM and great group of players.
Equally, as much as I like Ars Magica, I was in on saga that was terribly boring, due to the opposite situation...