It really depends on your priority.
If you want great speed and coverage in both rural and urban area then go with Verzion or ATT.
Between these two if you want to send your phone to Nepal when you get a new phone after two years of contract then go with ATT.
But if you live in urban area and rarely go to rural places and most importantly if your priority is the cost of the plan then definetly go with Tmobile. You can get same service plan for half the price of ATT and Verizon.
Tmobile does have coverage in rural places but not as much as those two.
Check their websites and tmobile's to see the coverage around your city/state.
Best of luck!