There is no simple answer to that, since the quality of service varies as you move across the USA. In any given area, one or two of the major wireless providers will have adequate service while another probably does not, but which one is best varies as you move around. Many travelers subscribe to two different services to increase their chances of having good service wherever they go.
The big wireless names in the USA are Verizon, AT&T, and T-Mobile but there are also several independent wireless services that purchase and resell access to those major networks.
In very general terms, Verizon has the broadest coverage because they serve more rural areas than AT&T and T-Mobile, but that doesn't mean much if they aren't good where you happen to be on a given day. In terms of the number of subscribers or other measures of nationwide coverage, the big three are pretty much alike.
If you are highly dependent on wireless for communication & entertainment (can't live without it!), then two service subscriptions are your best choice and worth the extra cost. If you can live without it for a night or two, I'd pick one (maybe Verizon), or one of the independents that use their network, and go with that.