One of the problems with answering this question is that there is so much fantasy out there, and most of it is really bad. Because there is so much out there, though, peoples' preferences will vary widely. Below are a few recommendations from me, but my taste lies in books where the fantasy elements are subtle, but the worldbuilding is thorough and the story is atmospheric.
You could always start with the classics:
--Lord of the Rings (Tolkien)
--Gormenghast (Peake): this is a bit different to those above, as the fantasy elements are more subtly done. There is no magic or battles, or any of the things you'll find in other stories, so it might not appeal to many folks. It is, however, in my opinion, the best written of the classics, and the most imaginatively and evocatively realised.
I don't know the Mistborn series, but the Liveship Traders implies you like stories that unfold as sagas (not that there's anything wrong with that). Have you read Robin Hobb's other works of the Realm of the Elderlings: The Royal Assassin Trilogy and The Tawny Man Trilogy? There's also a recent series that she's working on (3 or 4 books published so far) also in that world, and a trilogy (I think it is called the Soldier Son trilogy) that is set in a different world with overtones of American colonial period (but also strong fantasy aspects). Lastly for her, you can try some of her works by her pseudonym, Megan Lindholm (she wrote two relatively small books about a shaman's apprentice in a pre-Colombian American culture). I personally get sick of Robin Hobb rather quickly, although I acknowledge she has some mastery of her art.
Ursula Le Guin is a great fantasy writer, and her Earthsea Trilogy is now a classic in its own right. It definitely comes highly recommended, although be aware that it also has a somewhat subdued (i.e. slow and subtle) aspect to it. I believe Le Guin's parents were anthropologists, and so the worlds she creates are often very culturally detailed. She's also done a few sci-fi works you may have encountered.
Diana Wynne Jones was a very prolific writer. None of her books are very serious or epic, but I find them really good to just settle into. Enjoyable, well-written, sometimes sad and sometimes funny. She also liked playing around with fantasy cliches, which can be enjoyable (also, for the humorous side to fantasy, it is hard to miss Terry Pratchett).
Cornelia Funke has written some good fantasy works: Reckless and Fearless both take place in a fantasy version of Europe, but one in which the modern age is starting to catch up.
Tanith Lee has a large number of engaging books you might want to try. A trilogy she wrote starting with Black Unicorn is very good (and you don't have to be a unicorn-fan to appreciate the series).
A recent favourite writer of mine is Frances Hardinge (Fly By Night being a great starting point). She, like Le Guin, enjoys inventing cultures, and seems to take great pleasure in exploring how worlds different to ours might work. Her attention to detail is magnificent.
Lastly, I'd like to mention One Hundred Years of Solitude, by Gabriel Garcia Marquez. It isn't really fantasy (it belongs to a genre called magic realism), but it is one of my favourite books, and it seems appropriate to mention it now, because Marquez has just passed away.
I don't know if any of those titles help you out. I, too, would like to see what other people recommend, and why.