Depends on what you want.
Book of the Riteous is the best read of any RP book ever. It also has some great rules stuff in there. I cannot recommend it strongly enough, if you want a great read.
Chaositec has some really, really innovative stuff in there. It has great fluff, and great rules. But, you have to want freaky, psuedo-science/far realms stuff in your game.
Ptolus might be the single most impressive book in RP history. But, I have not bought it yet. I've come close twice, including in pdf last night. I've read parts of other peoples' copies. It is comprehensive, it is well written, it is well illustrated. I think you could use this forever and not use half of it.
Lost city of Barakus offers a great city. I like this book a lot, but its scope is not comparable to any of the above items.
I voted Ptolus, even though it is the only book of the four above I don't actually own.