It really is almost hard to go wrong with a pannier. The best advice I could give is to avoid bags made by companies that don't primarily make bags. Any company that bases their business on panniers, messenger bags, etc will have a good bag. If you are really interested in specifics, especially carrying capacity, durability, etc you might want to do a search over in the touring forum. They really get into their panniers over there.

My other thought is to be careful to not rule out a pannier due to high or low price. There are great bags at just about any price point (I use a set of Axiom commuter panniers that I paid all of $30 for new 2 years ago).