I like UPS the most, but not by a lot.
Here is my experience:
USPS doesn't have live tracking. The tracking has 2-3 entries with no updates. Pick up, a scan at a hub, and delevered. I don't like to ship anything too valuable. Mis-delivered packages are common. Insurance and tracking are not that good. UPS and FedEx ground is often cheaper on something that ways several pounds. For under a pound you can't beat the price and they are a strong part in my business model.
FedEx ground is terrible, but does have some strong points. They make it accross country 1 day faster than UPS. If you have a lot of claims they will make you send in a sample for testing. If you pass FedEx will continue to obey your claims. They break a few more packages, but to get resolution on the claim takes longer than UPS. Sometimes FedEx might break a package and it ends up in a black hole. There service via phone is pretty good. Shopping cart integration is very weak. FedEx express is strong. FedEx ground is delivered by subcontractors of FedEx; the driver may actually own the truck. Or a guy might own a few FedEx truck routes and hires his own people to make deliveries. I think that is the weakest link.
UPS is my favorite, but slightly. The web integration is the best. The drivers are better to deal with. As someone mentioned they don't really honor there claims but say their B.S. charter states that there must be 4" of packaging between the contents and the box which is unreasonable. They break less packages then FedEx did. I also got notifications of problems should they arise. They hide behind a third party for claims investigations.