I'll say it loud because nobody pays attention to this book:
Masters of Arms
This is THE book for warrior types AFIAC. It cuts the crap.
Other than that, I think with a few exceptions, I get more use out the WotC classbooks than any third party classbook.
Quintessential Books - to date, I think I like Wizard, Rogue, and Monk the best. Fighter is useful for the combat system and maybe some of the option rules, but not to hyped about the character rules.
Mindscapes by Malhavoc is THE classbook for psions.
Beyond Monks is a nice book for monks and has its own martial artist class (though to be honest, I rather like the updated version with the options to not use the martial artist class. As most supplements assume that the monk is the 3e martial artist, it seems to be the most productive way to approach it.)
Path Books - the actual prestige classes are pretty good, and I was especially happy with the prestige class selection for monks and barbarians in Path of the Sword... its WAY better than the corresponding WOTC books on those points. Also, Path of the Sword has some great supplemental material for fighters. However, I am not a fan of the XP-cost schools in any of the books, and the additional supplemental material in Path of Magic pales compared to similar material in the Quintessential Wizard.