Sharn: City of Towers positively *oozes* flavor. It's an incredibly dense book that helps to set the tone for the setting. True to its title, it focuses on Sharn, but even if you're not going to spend much time there, it discusses some other important setting bits, such as the Code of Galifar (legal system).
Races of Eberron has some great rules bits; excellent feats, racial substitution levels, and PrCs in particular. With the exception of the *fantastic* Kalashtar chapter, the fluff chapters range from uninspiring (Warforged, Shifter) to downright damaging (Changeling "Passer/Becomer/Reality Seeker" nonsense, I'm looking right at *you*).
Faiths of Eberron does a fantastic job of fleshing out the beliefs of the Sovereign Host; I like the rest of the book quite a bit as well.
Dragonmarked has great options for 'marked characters and the abberant 'marked.
I own all the rest, with the exceptions of Secrets of Xen'Drik and Player's Guide to Eberron (less gaming funds these days than I'd like). I've liked them all so far, but Magic of Eberron has excited me the least.
How much exposure to/familiarity with the setting do you have so far? Do you have any ideas as to particular parts of the setting that you or your players would enjoy most/would like to focus on? That could drive your purchases as well.
In any event, make sure that you take advantage of the
Dragonshards articles as well - there's some great stuff there! Blades of the Quori, Gnomes of Zilargo, Heirs of Dhakaan, and Daughters of Sora Kell are among my favorites.