That's like asking "Which paintings are art?"
If you consider games as a medium a form of art, then you consider all games as art. I'm not saying that most of them can't be terribly done pieces of art (in the same way that a few lines of different coloured paint on a canvas is still art, even though it's probably not that great), but nonetheless they are still art.
EDIT: If it's just "good" examples of games as art you're looking for, though, then I have to agree with the many people who beat me to the punch suggesting Okami.
If you consider games as a medium a form of art, then you consider all games as art. I'm not saying that most of them can't be terribly done pieces of art (in the same way that a few lines of different coloured paint on a canvas is still art, even though it's probably not that great), but nonetheless they are still art.
EDIT: If it's just "good" examples of games as art you're looking for, though, then I have to agree with the many people who beat me to the punch suggesting Okami.