1. Player's Handbook - Really transformed character creation for D&D in many ways (for the better). The named, iconic character illustrations by Sam Wood and Todd Lockwood really evoked the genre. In most ways 3rd edition is my favorite version of Dungeons & Dragons even though I'm glad it was replaced by 4th edition (going on 5th edition).
2. Dungeon Master's Guide - Adding levels to monsters really intrigued me. I liked the art that was re-imagined from AD&D such as Wayne Reynolds' skeleton attacking Alhandra on page 114. The magic items were cool with good illustrations. I was constantly envisioning ways to leverage the immovable rod.
3. Monster Manual - This had the best cover of the three core rulebooks. Dragons by age charts were a great idea in principle. The book was filled with art that I liked, which matters to me.
4. The Forge of Fury - Perfect for plagiarism. Over and over again, many DMs stole these maps and some of the room descriptions for games I played.
5. Sword and Fist: A Guidebook to Fighters and Monks - Full of awesomeness and written by Jason Carl (one of those DMs that recycled The Forge of Fury maps into other settings). What more needs to be said? The black & white drawings in this book almost all rank among my favorite D&D images. The feats and prestige classes rocked, too.
7. Magic of Faerun - Dead magic, wild magic, the shadow weave, spellbooks inscribed on steel pages, and unusual metals for weapons & armor were all interesting concepts that I reuse constantly. I recall that most of the broken prestige classes seemed to come from this book but also utility feats like energy substitution.
8. Epic Level Handbook - I've mentioned elsewhere that I'll never use these rules. They amplified the heroic art in order to make it feel epic and I think they succeeded. Wizards of the Coast, please realize that we want our characters to feel that cool even at 5th level and project that same amount of awesomeness even in art orders for levels 1-10. I just love to browse this sourcebook.
9. Savage Species - Above I mentioned that the DMG made this list in part because of adding levels to monsters. A missed opportunity for Savage Species would have been to provide level progressions for three monsters from the Monster Manual (dwarf, elf, and halfling). Level progressions for dwarf, elf, and halfling would have been a fun nod to D&D Basic and old school fans as well as easily brought into 3rd edition NPC communities the vibe those D&D Basic classes established for each race. D&D 4th edition tried to fix problems with Savage Species and update it but I prefer the Savage Species variant to what has been done since.
10. Races of Faerun - The cover illustration is kinda DC Comics but despite that still stands out as one of my favorites (shown below; by Greg Staples). This sourcebook provided a little variety for the player character races, which was much appreciated.
That's my list. What would you have included differently?