What are your needs, what material do you prefer, what else do you plan on carrying in it? There isn't a single best solution for everyone. Personally, I have a LL Bean bookbag with a padded laptop pocket in the front of it. I am a student and it holds my books as well. I couldn't be happier with it. After nearly 2 years of abuse it looks like new and my laptop hasn't had a problem from it.
Brian