I have just re-reread this after a gap of six years, and can confirm that I rate it as one of the best 'children's' fantasy books of all time - to be set alongside Wind in the Willows, The Hobbit, Narnia and Harry Potter (my personal pantheon).
Like each of these it is unique, a one-off - and clearly written under some kind of inspiration.
Also, Watership Down has that multi-valent richness and depth of the best books in this genre - including a vivid, convincing and beautiful incorporation of the rabbit's religion and spiritual life (and some of their language!); which is, I think, better-done than in any of the other-mentioned books.
Watership Down has great characters, adventures, love, battles, natural beauty, humour, mystery and high seriousness, and lots about the life of bunnies! - a complete and rounded sub-created world; but to pick these apart and present them separately would be to distort and misrepresent the feel of the book.
Certainly, it is a work of genius.