Star-Wars d20 revised edition is the book that I do find the most beautiful of all my d20 books. However, I almost never play d20 Star-Wars.
I also much like d20 Modern which is self contained game in one unique book. Much like Menace Manual too (excellent monster book, nice illustrations, very well done).
There is many books that I do use, and one of my favorite is Grim Tales. However, I don't mention it because visually it isn't very exciting (black and white, few illustration not of over the top quality, simple layout).