I liked the the background in AO/AE. Especially in AE, where I found myself stopping whatever I was doing and gawking at the background.
My top 3 backgrounds:
1. The Slig Barraks. Especially the huge orange-tinted window in the War Room. You only get to glance at it for a second, though.
2. Mosaic Lines. Look at those huge statues!
3. Necrum.