Probably "Shadow of the Colossus", the sense of scale that the game has to offer is still pretty much unmatched. It also did some very impressive things with fur, animation and HDR that I haven't really seen replicated anywhere in that quality.
Sure other games have more polygons and smoother framerates, but almost all of them fail at actually creating a believable world with that, the illusion they try to create with their pretty painted corridors falls apart very quickly the moment you bump into yet another invisible wall. So while there might be games one could consider to have better graphics in a static screenshot, I haven't yet seen anything that really matched SotC when actually played.
Sure other games have more polygons and smoother framerates, but almost all of them fail at actually creating a believable world with that, the illusion they try to create with their pretty painted corridors falls apart very quickly the moment you bump into yet another invisible wall. So while there might be games one could consider to have better graphics in a static screenshot, I haven't yet seen anything that really matched SotC when actually played.