The whole point of messenger bags are so that you can open them and take items out quickly without removing the pack, hence their use among bike couriers and messengers. Nowadays, a "proper" messenger bag is designed to be worn high on the back while cycling so the pressure is spread across the back instead on one shoulder.
A good messenger bag will utilize snap closures or velcro for easy access. Most of the bags listed in this thread are shoulder bags, but not messenger bags, with belt buckle-style closures.
If you still want a messenger bag and not JUST a shoulder bag, popular companies are:
Chrome,
Mission Workshop, and
timbuk2. It is worth noting that Mission Workshop and timbuk2 each have a lifetime warranty.