If there ever was an underrated/underappreciated masterpiece, it’s this one, Tad Williams’ massive 4-volume epic Otherland. Memory Sorrow and Thorn is considered his magnum opus for good reason but imo Otherland is almost on par with that series. And in general, it’s one of the best sff series I’ve ever read.
Now it’s definitely leaning a bit more on the scifi side considering it’s based on virtual reality, but the overall narrative definitely feels very fantasy-ish. The worldbuilding in this series is simply phenomenal and I’ll put it up against the best in the genre in that aspect - it’s just as good as ASOIAF or Wheel of Time when it comes to creating imaginative, unique and downright crazy concepts and premises. It’s simply Tad Williams flexing on them hoes with how creative and imaginative he is.
The plot and characters are very compelling as well and the books just accomplish what the best fantasy stories usually do - completely immerse you into an alien world with its own unique rules
My sff tv adaptation dream is actually a multi season series based on the Otherlands books. please read them - you won’t be disappointed.