I'm currently re-reading
Joseph and His Brothers, a 1200-page heavy-weight monster of a book by Thomas Mann (himself a heavy-weight monster of an author).
Do I recommend it? Yes, but not for everyone. The preface itself is one of the heaviest essays on theology/philosophy out there, but the whole book is crammed with brilliant ideas/wisdom and is a masterpiece of literature (but it's heavy and the story moves fairly slowly).
Something lighter along those lines would be Narziss & Goldmund, by Herman Hesse - also heavy-weight, contemporary and compatriot to Thomas Mann, but capable of a lighter style!
But yeah, I love to read (and not just German literature... those are just the examples that spring to mind right now out of the countless ocean).
To answer your other questions...
What is your most favourite book and why?
What's your favourite movie? If you read any significant number of books, naming a single favourite is impossible - unthinkable, even.
How many books would you estimate you read a month?
It really depends on the books and the time of my life... sometimes I've finished a book in 3 days, but other times it's taken me months to get through a single book (such as the Thomas Mann brick I'm currently reading).
do you think all this reading is worth it?
I think life would be pointless without it. At the very least, it would reduce my mental freedom to not read books. I made an argument for that on my blog: http://inter-sections.net/2007/12/13/sapir-whorf-books-and-y...
would it not be better if you spent your time doing something else and why?
It really depends on what you're reading. If you spend your time reading Dragonlance novels (I've done that, it's nothing to be ashamed of, so long as it's a phase that passes), then yeah, you could probably do something better with your time. But books are still far and away the deepest and most fruitful repository of human wisdom that we have available to us. Sure, you can learn all you can learn from books, from your own life. But you'd be a fool not to climb on the shoulders of those giants who have come before us if all it takes is a few hours of comfortable, enjoyable time with a good book.