The USPS without doubt.
Most courier companies have limited offices in different countries, making collection of items from the likes of UPS, Fed Ex, DHL a royal pain in the arse. It may work for businesses, but not for domestic deliveries and make no mistake, most of them dont give a stuff about non-business deliveries. The money is with the big contracts from big businesses to other businesses.
DHL is a BIG no no. They have been bought out by the German post office and the shipping to europe now from the US is a bloody joke! EVERYTHING goes via the fatherland!!! ( How do I know this? Because Amazon.com use them, and now even the standard mailing items go via Germany and so take twice as long as standard mailing ).
Fed Ex? Bah! Invoiced a month AFTER something was delivered for customs charges-something you expect to be notified on upon receipt of the item! Customer service was very rude and lied outright to me when I demanded some explaination why I was not informed of any charge at the time, ( he thought I was an average joe, not knowing that I work for the UK postal service ).
All of the private companies 'dump' packages anywhere they can, ( doorsteps, neighbours, etc ), in order to maintain their delivery claims.
Do some research on your national postal service, take a look at what it offers to the public, and then decide if you would like to lose that.