Uhh have you been to most places? I'd say it's impossible to claim that either Verizon or AT&T are better in most places and pretty useless.
No, but I have been to many different cities across the US, often traveling with others not all on Verizon, and found my coverage to generally be superior than competing networks. And as I already mentioned, this becomes doubly apparent as soon as you get outside of a major city, where the difference often means Verizon==coverage, and anything else == nothing. (Let me be clear though that I don't want to be a shill for Verizon - they are an evil company with absurd pricing, but you can't argue with their coverage.)
where you actually plan on using your device makes a huge difference. Trying to generalize which one has better coverage is useless, he needs to figure out where he'll be living/working and talk to people in that area.
The OP asked for the best provider in the US, not the best provider for his parent's house corner bedroom in Appleton, Wisconsin. This suggests that he does not yet know where he will end up, or may be traveling across the US, and thus be in need of a good all-around-carrier. You most certainly can generalize that.