In this one, I'm going to talk about the movie, not the book. For those who haven't read it, it's a fantastic book, but certainly not a light read. The movie's a bit easier to get through, I guess.
Watership Down is one of those classics, both of book and film - even though it is only a cartoon. Honestly, in this case, I don't think it would have been done at all well if it wasn't one. After all, if it's a story about rabbits - well, they're a bit difficult to get to do your specific things you want them to do and such. And yes, for those unfamiliar with it, I know, it seems a bit strange. A story about rabbits. Bear with me.
The basic story is this: these rabbits are living fairly cosily in their den, but one of them (Fiver), who is somewhat prophetic, foresees that there will be death coming to the warren, and they all have to leave. Some do, some don't. The rest basically details their journey to this "Promised Land", as such, which is Watership Down, a great hill where you "can see the whole world" from. Oh, and their attempt to then get some females, because they didn't bring too many along.
It's got some great plot elements to it, and some interesting animation when it comes to some of the rabbit mythology, particularly in relation to Lord Frith, their "god" of sorts. It's a good drama, which has some great elements of adventure, but also a bit of comic relief, particularly with Kehaar the gull.
If you haven't seen it, do. Seriously, do. It's an amazing film. And if you haven't read the book, I'd suggest that to you as well, particularly for the more avid readers. It details some of the mythology and language a lot further, which can be quite interesting.