Discussed regularly in the Technology Forum. The consensus of many who travel extensively is that Verizon's network has the broadest coverage, but the best deals for buying no-obligation data service might be re-sellers like Millenicom, depending on just how much data you need and whether you might be buying other services too.
Be aware that Millenicom re-sells services from more than one carrier, so one plan might be on Verizon, and different plan on Sprint.
From my viewpoint, Verizon doesn't work because they aren't licensed to sell me service, and I live and travel in some of the holes in their network. People who travel the Interstates from metro area to metro area may have never found the holes.