Most: Gee, um Monsternomicon, Oriental Adventures and let's say Call of Cthulhu. Given that we're excluding the PHB and noting that I've never run an Iron Kingdoms, a Rokugan or a Call of Cthulhu game in my life.
Oh, and Grim Tales.
Least: Let's include Cry Havoc and The Path of the Sword. Fine books, no use for 'em.
I've gotten plenty of use out of both Deities and Epic, actually. But then I inflict my non-epic, non-divine characters with some pretty stupidly powerful bad guys.