PC Magazine has done an exhaustive test of mobile Internet speeds and relability throughout the United States, and has found “AT&T was the fastest (albeit the least consistent) of the four nationwide carriers.”
Not only did AT&T score highest overall, it also beat out all other carriers in 3 of the 4 US regions (covering 18 cities) that PC Magazine tested in. The study also tested 4G networks as well, but did not test voice calls (the main complaint of AT&T/iPhone customers).
Here are the tables that PC Magazine put together to show the results of the tests:
Top Image all charts: PC Magazine