I think the best bags are Timbuk2. They are very fashionable and utilitarian. Further, they were designed my actual couriers for comfort.
A few words of advice:
1) Buy the bag off eBay or Craigslist.
2) Make sure it comes with the optional shoulder pad.
3) Make double sure it comes with the cross-chest stability strap.
4) Decide if you want the optional handles.
These bags are fairly expensive if purchased directly. A maxxed out laptop messenger might run you $100 to $150. They are much, much cheaper on eBay, etc.