I thought the Twilight Realm in Twilight Princess was awesome. Or, well, whenever Hyrule was enshrouded in twilight. The Shivering Isles expansion in Oblivion also looked crazy, especially the way they did the sky at night. It was fucking gorgeous. Some parts of Final Fantasy 13-2 looked nice too, like the forest area.
Sonic the Hedgehog games tend to have beautifully designed landscapes. Like grinding rails in space. Fuck yeah. There are others, I just can't remember them all.