I hate to admit it, but brown is the best. I should know, I 'm an investor in a shipping consultancy that helps mid-size companies re-negotiate their UPS / FedEx / DHL contracts.
However, it really depends on the nature of your business. If you're sending less than 200 1-3 lbs packages to 100% residential locations.....USPS provide you with the most value. Furthermore, despite what "Twinge" said, the post office is the BEST at rural residential delivery. They are the only carrier that delivers to every US address. (UPS is around 97%)
However if any of the following conditions apply to your business, you might want to consider UPS.
1. You ship more than 50 packages a day and need a more robust shipping system.
2. You customer service person sends a significant amount of time on USPS tracking packages.
3. Your customers need more information regarding the status of their packages.
4. You drop ship any or a majority of your products.
5. Your business is steadily growing. (Get a UPS contract in place so you can establish some shipping history and secure discounts that you can grow into.)
6. You prefer signatures on all your shipments.
7. You want to start shipping C.O.D - "Collect (check) on Delivery"
8. 20% or more of your shipments are Expedited Air (overnight, second day)
Good Luck!