My personal top list of Hesse's books would be:
1. Steppenwolf
2. Demian
3. Siddhartha
4. Journey to the East
5. Das Glasperlenspiel
6. Narcissus and Goldmund
7. Knulp
8. Peter Camenzind
9. Klingsor's Last Summer
10. Rosshalde
Although to be honest, I read them all a long time ago, and if I re read it again, the order would probably change. Some of Hesse's books definitely have a certain critical age bracket when you should read it to enjoy it the most, in my opinion...