In my experience, T-mobile had a lot of no-service areas. This included areas where they were supposed to have coverage. That was a problem in my regional days because we went to some small towns.

ATT had good coverage but the data was essentially useless at a busy public area like a hub airport.

Verizon has great coverage and pretty good data. Data can get a little slow at a big airport on a busy day, but there's probably no avoiding that until the infrastructure gets upgraded a couple more generations. Voice calls always work, never had one bounce to VM that I can recall.

My t-mobile and ATT experiences were years ago, YMMV.