I'd echo Buttercup's comments as well. It's an amazing book.
However from your list, I voted Fiend Folio, but I think the BoVD is extremely good, and I like 3.5 a lot too. AU holds next to no intrest to me as I still like and enjoy the DnD fantasy archtypes. Stronghold Builder's guide is kinda cool but not essential, same with savage species (and if you're goign to upgrade to 3.5, maybe don't bother) Ghostwalk also looks kinda dumb, and I've heard bad things about Arms and Equipment Guide, but it looked like it would have useful stuff to me (it's not a list of armements, it's got rules for vehicle combat, hirelings and the like).