This is bit difficult question for me as I haven't read all the books… And among those which I have read, there is couple of tough alternatives to choose from.
But still, I think the first experince is always the best. So I'll say my favorite book is The Bellmaker. It was the fisrt book of the series I read and my first step into the Redwall-world. I picked the book for my school assigment and I choose it because of its cover: it was an eye-catcher, the red caped mouse-figure was so mysterious I had to find out who he was xD (and Joseph the Bellmaker is still one of my favorite characters)