I like my Timbuk 2 classic large messenger bag for commuting (and general riding, day tours, etc.) I suppose I have more of an "aggressive" riding posture, and it fits nice and flat on my back while riding. I recently pinned a big flashing slow vehicle triangle to it for night riding. I've commuted in some pretty nasty stuff, and haven't lost a laptop...yet!

It's been a good bag for light hauling as well, but I've found a trailer to be much better for heavy/bulky loads.