Largely depends on how the coverage is in your area. Each carrier has it's strengths and weaknesses. Sprint has rotten CS, while T-Mobile's is an award-winner...but T-Mobile's coverage tends to suck outside of major metro areas, compared to Verizon and AT&T...but Verizon can be extremely limiting with locked down phones and walled gardens, while you get more freedom with AT&T...but AT&T service tends to be a bit more expensive.
Then of course, you might have a smaller local or rural carrier in your area. They have advantages too, as their coverage tends to be EXCELLENT in comparison to most others, but they might not have the features of the major carriers.