DrDude, at the moment I don't think anyone even knows what the best one is. Here's what I understand so far:
USMLEworld: Cheapest with the least amount of questions. Good questions, very, very difficult and multi-layered. Sometimes the questions are poorly worded. Good question reviews.
Qbank: The most expensive by far but the old standby. Some people claim that its questions focus too much on memorization, others say that's exactly what they need to drill it into their brain. You can't go wrong with qbank.
USMLErx: Haven't heard much about it, made by first aid, if your goal is to memorize first aid backwards and forwards I think this would be a good companion.
Personally I'm using USMLEworld, Qbook, and Robbins Review of Pathology. But I don't really feel like I have enough time to go through everything (~6 weeks). Oh well...