I can't believe no one has mentioned the Seagull Black Bag. Definitely the best bag for the money. I don't have one, myself, but every review I've read on these and other forums has been very positive.

I use a Pac Designs Ultimate and it is amazing, but too expensive for some to justify (understandably). From everything I've read Pac, Baileyworks, Seagull, and Under the Weather seem to be the favorites.