Which has the best coverage is not as important than who has the RIGHT coverage.
Verizon has the best coverage.. But.. I'm not a fan of Verizon, and other companies are making up ground fast.
But some of the places I need coverage.. Verizon has nothing.. but T-Mobile works
Other places no T-Mobile or Verizon, but Sprint (And it's derivatives like Virgin Mobile) work.
I should point out that there are a lot more cell companies than there are service providers.. Virgin mobile buys from Sprint. Trac Phone from assorted companies. Smart Phone, I do not know. But far as I know those above are the only nationwide providers.
Oh the absolute best data coverage would be Hughesnet.. but that is not cellular, it's not mobile (Portable yes but you have to park) it is slow and there is a 1/2 second minimum ping time.. It is 2-way sat
Excede is faster and less ping time,,, but.... is not compatable with mobile life style since they spot beam.