The fixed gear/track forums have many discussions about what the best messenger bag is for a cyclists. There are some fans of Timbuk2 but also many who do not like them. I think Timbuk2 is the most commonally available but not necessarily the best.
I bought a Chrome after reading many of the threads and looking at the various websites. I am very happy with my pruchased 2 years later. I commute with my Chrome Metropolis everyday in all kinds of Cleveland weather. It is holding up well and keeps the contents dry. Plus it is comfortable and can hold a ton.
If you want something a little more unique there are several smaller manufacturers that will make custom bags with your choice of colors and even designs.
Good luck in your search.
Craig