Basically, from 1969 to 1980, he had the most unbelievable streak of great albums of any artist in history (even Young Americans, which I'd give about a 7/10). The Berlin Trilogy he did with Eno is about as experimental as a pop star can get.
My rankings:
1) Low
2) Station to Station
3) Ziggy Stardust
4) Heroes
5) Aladdin Sane
6) The Man Who Sold The World
7) Hunky Dory
8) Lodger
9) Scary Monsters
10) Space Oddity
11) Diamond Dogs
12) Young Americans