Check the largest US carriers to get a better idea of which no contract provider will work for you. Almost all NCP/MVNO’s resell service from these networks in the USA.
AT&T Coverage Map (with roaming)(GSM, 3G, 4G, LTE). “Good” or “Best” coverage means you should consider an AT&T-based service.
Sprint Coverage Map (CDMA, 3G, 4G, WiMax, LTE)(Virgin Mobile’s website, but Boost Mobile and other Sprint MVNO’s are identical in coverage). However, coverage may vary based on whether or not your carrier allows roaming (for example Ting roams, but Virgin and Boost DO NOT roam). Good coverage means you should consider Sprint-based coverage.
T-Mobile Coverage Map. (GSM, 3G, 4G)
If you get “Very Strong” or “Excellent” coverage, you should consider a T-Mobile based service.
Verizon Coverage Map (CDMA, 3G). Be sure to tick the “PREPAID” option for most accurate results. Verizon prepaid networks do not support 4G or newer technologies at this time.
Otherwise, coverage on this network means you should consider Verizon-based coverage.
There exist community-run websites allowing you to check coverage on more than one network at once. Why not have a look at those, too?