This really depends on where you live.
My last job provided a Verizon phone. I had to stand by my window to get a signal.
I'm in the St. Louis area, so it's not like I'm in the middle of nowhere.
AT&T has the best coverage and now pricing where I'm at (I get 5 bars at home).
I was considering switching to T-Mobile, due to cheaper costs. Their high-frequencies (1700/1900/2100) are just terrible where I work (NO in-door coverage).
Since AT&T has dropped subsidies, my monthly bill is as cheap as it would have been with T-Mobile, but I get to keep my coverage!