The best bag I have found, by far, is the Tristar convertible bag by Tom Bihn at tombihn.com . It measures 19"x13"x8".
It is rugged, well designed, and good looking, and it carries well with either handle, backpack straps, or the ultimate shoulder strap. It is not cheap, but the external pockets keep everything organized and accessible. You can order (at additional cost) the Cache which snaps into the center compartment to protect your laptop. I find that the Eagle Creek ultra light packing cubes are a better value for keeping things organized within the pack. The Tristar came with metal zipper pulls, but I cut them off and installed the nylon corded pulls which were included with the pack. It holds 2000 cubic inches of stuff. If you need more, you can step up to the Aeronaut, at 2700 cubic inches, which is actually cheaper, but it is bigger.
I have traveled several times to Europe with this bag, and as a personal item I selected the Tom Bihn "Co-Pilot" shoulder bag. Same quality, same high price point, same durability, but serious luggage for serious travelers. If this sounds like a commercial, I apologize, but I love the bags. Lifetime warranties on both.
One important note. If you are using the bag as a backpack, take off the shoulder strap. I left the shoulder strap connected while using it as a backpack as I walked across the Promenade de Anglais in Nice after getting out of a taxi. I accidentally stepped through the dangling shoulder strap and tripped in the middle of the street, landing hard enough to break my wrist. I soldiered on, however, binding my swollen forearm with a velcro fastening nylon brace and fortifying myself with copious quantities of wine and cognac. Broken bones will heal (sometimes with a little surgery) but memories are forever, especially memories of France and other exciting places.