Naming just a single book is near impossible (and the book I’d name would likely have to be TLoTR), so I’ll rather list a few that are perhaps less widely known, but still very much worth it (if the genre is your thing).
Adolf Muschg: Der Rote Ritter. Eine Geschichte von Parzival
This one is likely only available in German. If you read German, this is a must, for the language alone, but it’s also a decent interpretation of Arthurian legend. If you are more into contemporary fiction, an alternative read would be Sax by the same author.
Dietmar Dath: Feldeváye: Roman der letzten Künste
Again, German only, I fear. Science Fiction. Hard to describe, really. It has a bit of Dan Simmons’ Hyperion Cantos, but less action-y and more philosophical.
Helga Glaesner: Im Kreis des Mael Duin
Standard fantasy fare. Again, German only. Not too bad and it’s a single volume that does not needlessly drag on and on. If historical fiction is more your thing, try Die Safranhändlerin instead.
Now something for our international audience:
Brian Catling: The Vorrh
Historical fantasy or fantastic history, set in colonial Africa. Brilliant writing. It’s the first in a trilogy, with the final installment due in July this year.
Patricia McKillip: Cygnet
More fable than fantasy. If you’re fed up with the likes of A Song of Ice and Fire, this one (or any other of hers) manages to entertain without people dropping dead left and right. If you read a couple of her books, you’ll notice that they more or less follow a similar pattern, so stop before it gets too boring.
Peter S. Beagle: The Innkeeper’s Song
You’ll probably know him for The Last Unicorn (shame if you don’t), but that lovely fantasy story has a fine style and is quite enjoyable (and over far too soon).
Valery Leith: The Company of Glass
Fantasy, but of a weird kind. In a good way. First of a trilogy (in Germany, only the first two volumes were released, so better to read in the original).