I'd have to chip in and say I think it's the greatest novel ever written as well. It's hard to explain why, though, if you haven't read it, because it's not necessarily a plot-driven book.
It's more the fact that if you read the whole thing, you feel that you've lived through the Russian/French wars. So all the little details, which seem incidental at the time, add up to a massive picture of life.
Anyway, if you still haven't read it and you want to have a go, I'm conducting a challenge on my blog at the moment to read through
War and Peace in one year by reading a chapter or two a day.
There's about 11 of us doing it, but we only started on 1 July, so you can quickly catch up if you want to join in.
The starting post that began it all is this one:
Read War and Peace - in just 10 minutes a day! The Relentless Pursuit of Cold Shivers
To everybody who's already read it - you already know how great it is - and I can completely understand reading it three times.