I like the "City" books - Highthrone, Skraag, Stormhaven and Stonebridge. (I think there were four.) They are interesting places to stick in your campaign world.
I'm not a fan of the Quintessential books, since most of it seemed unbalanced to me (or "balanced" by making an overpowered sorcerer feat have Cha 19 as a prerequisite, so only min/maxers could take it, resulting in "the rich getting richer")
The Seas of Blood stuff was evocative; I think "Classic Play: Book of the Sea" is a compilation of it, but I'm not sure.
"Encyclopaedia Arcane - Enchantment" had some pretty good fluff, and some flavourfull spells. I particularly liked the "Camaraderie" spell, which makes people you have already charmed feel friendly and co-operative towards each other.