Mobile Internet access is a highly-contingent matter, and varies as much between individual cell sites as between carriers.
That noted, a new study of mobile Internet access speeds among the largest four U.S. mobile providers found that Verizon has the “fastest 4G network” on an overall national basis, based on its performance in 16 of 30 cities, but all of the suburban and rural areas where tests were conducted.
In various areas another carrier can have the fastest network on 4G or 3G networks. AT&T, for example, has the fastest 3G network, nationally.
T-Mobile US performed best in Northwest and second best at Southwest locations, at least in urban areas.