Your Top 5/10 Favorite WotC Supplementary Rulebooks

[Thomas Percy]

Tome of Magic
Complete Warrior
DMG2

 

[Prophet2b]

1) Tome of Battle
2) Complete Adventurer
3) Complete Warrior
4) Unearthed Arcana
5) Player's Handbook II
6) Magic Item Compendium
7) Complete Scoundrel
8) Spell Compendium
9) Complete Arcane
10) Complete Mage

 

[Odhanan]

Unearthed Arcana
PHB2
Book of Nine Swords
Spell Compendium
Magic Item Compendium
DMG2
Libris Mortis
Complete Arcane
Fiendish Codex 1
Frostburn

These were actually used in-game. Great stuff all around. As a side note, I would have loved to include Tome of Magic in there: the ideas are absolutely awesome. What I do not like about them is the lack of balance of the classes concerned (meaning: they are underpowered, particularly the Shadowcaster and Truenamer. You can find great advice from Ari Marmell to "fix" the Shadowcaster, and the Truenamer can easily be fixed on its own, but that feels like the material wasn't finished in the book, to me).

 

[Pants]

1. Fiendish Codex I - And absolutely awesome resource for the demon-heavy campaigns that I run. Great monsters, great flavor, awesome all around.
2. PHBII - Best. Crunch Book. Evar. Gives a much needed boost to high level fighters, retraining is pretty cool too
3. Lords of Madness - Excellent reading, good monsters, great fluff.
4. Complete Adventurer - It gets used pretty much all the time
5. Fiendish Codex II - Not as good as FCI, but still a nice resource
6. Tome of Magic - Great art, great ideas, questionable execution
7. Expanded Psionics Handbook - Psionics finally done right
8. Unearthed Arcana - Great source to mine ideas to modify your homebrew, not used that often though
9. Draconomicon - Fun to read primarily
10. Magic Item Compendium - Good resource, though the layout could use some work and I'm not entirely sold on the repricing thing

Honorable Mention: Tome of Battle - Gets HM just for being a really cool system whether or not its overpowered or underpowered

 

[Olaf the Stout]

I'll have to come back and add some others in later but off the top of my head, the first 3 books to spring to mind are (in no particular order):

1. PHB 2
2. Spell Compendium
3. Magic Item Compendium

Olaf the Stout

 

[Greg K]

1. Unearthed Arcana
2. Fiendish Codex I
3. Dragonomicon
4. Book of Vile Darkness
5. Lords of Madness
6. Stormwrack
7. Complete Warrior
8. Fiendish Codex 2
9. Monster Manual 2
10 Fiend Folio

 

[MrFilthyIke]

In no particular order:

Fiendish Codex I
Fiendish Codex II
Unearth Arcana
MMIII
Tome of Magic
Book of Challenges
Dungeonscape
Stormwrack
Frostburn
Sandstorm

 

[Jhaelen]

1. PHB2
2. Tome of Magic
3. XPH
4. Weapons of Legacy
5. Tome of Battle

 

[Particle_Man]

1. PHB II (love the 4 new classes and the high-level feats)
2. Expanded Psionics Handbook (yay! Augmentable powers!)
3. Tome of Battle (cool!)
4. DMG II (Mob rules!)
5. Complete Arcane (Warlock!)
6. Complete Adventurer (Great feats! Scout!)
7. Complete Warrior (Tactical Feats!)
8. Complete Mage (Reserve Feats!)
9. Complete Scoundrel (Skill tricks! Good class combo feats!)
10. Unearthed Arcana (Lots of ideas, here!)

 

[Michael_R_Proteau]

For me...

1) Unearthed Arcana
2) Heroes of Horror
3) Heroes of Battle
4) Spell Compendium
5) Player's Handbook II
6) Complete Adventurer
7) Complete Scoundrel
8) Fiend Folio
9) Frostburn
10) Dungeon Master's Guide II

Just off the list-I like a lot of stuff from the Miniatures Handbook (Healer, Marshall, some of the feats, spells, and magic items) but not the miniatures specific stuff in it, which keeps it from the top 10.
-M