What I typically use:
WotC
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Player's Handbook (3.0e) - I treat the 3.5 SRD/books as ill-thought out errata as from an ill-informed customer support agent, much of which is suspect and some of which is crazy, and only some of which is worth paying attention to.
Monster Manual (3.0e.)
Manual of the Planes - Good templates, useful information, a few monsters
Third Party
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Tome of Horrors (Necromancer)
Arcana Unearthed (Malhavoc)
Book of the Righteous (Green Ronin)
Shaman's Handbook (Green Ronin)
Beastiary - Predators (Betabunny)
Hot Pursuit (Adamant)
Classics
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1E AD&D DMG - Still use dungeon dressing tables, and still inspirational reading
Pathfinder
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Haven't seen a bad book from them yet, though also haven't been tempted into buying any of their books either.
The vast majority of 3e books to me suffer from uneven quality and far too little useful material for the price. I don't feel like I'm getting my money's worth from a monster book, if I'm only going to use 2-8 monsters from it in actual play the whole time I own the book. Most of the player's options books just have way too much broken material in them that leads to power creep and escalating wars of power inflation as the DM deflates CR or otherwise increases the EL by previous standards to keep up. This prompts players to respond in kind with more optimized builds. The plethora of mechanical options that only kick in at high levels (templates, high LA races, prestige classes, etc.) tends to skew player's to see high level play as the only satisfying play. Mutually assured enjoyment destruction is IMO insured.
The monster fluff books like Libris Mortis, Lords of Madness, and Draconomicom are really light weight in my opinion.
I love the idea of Template books, but in practice I find just customizing a monster by hand a less troublesome process than checking it against a template, and not only is it quicker (because everything I do is 'right'), but I get exactly what I want at the end of it. Besides, most of them feature a lot of templates which in practice I'd never use. A book that is mostly templates I'd actually use is rare.