Xenoblade has to take the cake for me. The novel structure of the world, combined with the superb art direction, led to some spectacular results. Each new environment dropped my jaw more than the last.
Here's a 14 minute environment porn video from the game (minor spoilers): https://www.youtube.com/watch?v=iAYBui5ghJ0
This is one game that I would really, really like to see remastered in HD.
This ^^^