Top 5, eh? Way easier than top 10 (the core 3/SRD notwithstanding):
Ptolus
Monte Cook's Arcana Evolved
Draconomicon
Complete Book of Eldritch Might
Libris Mortis
These are the books I've found most inspirational or never once thought "I can't believe I spent $XX on this!"
JediSoth
Ptolus
Monte Cook's Arcana Evolved
Draconomicon
Complete Book of Eldritch Might
Libris Mortis
These are the books I've found most inspirational or never once thought "I can't believe I spent $XX on this!"
JediSoth