The Primeval Thule Campaign Setting and it's supplements is far and away my favorite 3rd Party 5e product line. It was all written before there was an official 5e SRD and OGL, so the phrasing is a bit off from 5e standards, but the book is amazing anyway.
A lot depends on what you like, of course. Thule was pitched as "Conan meets Cthulhu," and that's right up my alley.
There are a lot of great things about Thule, and I just started running it!
