Verizon
I like knowing that I'll have fast LTE almost everywhere and inside just about every building. Unlimited data is a nice touch too.
What I think about the others:
T-Mobile - good promotions, good data service if you mostly stay in your metro. Terrible in rural areas and on interstates between cities. Lower price isn't worth 'E', 'GPRS', or 'No Service'
Sprint - terrible on all accounts. Usually the slowest network even in cities. Locked down phones, poor rural data coverage just like TMO.
AT&T - Competent network. Not quite as expansive as Verizon but overall somewhat tolerable, especially in the Northeast, not as much in the South. Doesn't offer a real unlimited data plan though.