DHL looks like the little guy here, but is the most reliable of the bunch: check out http://www2.isye.gatech.edu/~jjb/wh/package-race/2007/2007.html
DHL's success over Fedex and UPS is partly because they're not so little - DHL Express is owned by DeutschePost World Net, the second largest employer on Earth (after Walmart). That means big investment. DHL is huge outside the US, just not very well known here.

USPS is huge, but they should be after years of having been a government department. They had generations of taxpayer investment and no shareholders to report to, so got all the infrastructure, buildings, vehicles, and government protection they needed without any need to make any of it profitable. We all hear about carrier issues, but USPS has the service down to a science.