Unfortunately, we only have 4 real options in the US. Everyone else is just piggybacking on one of them.
I like to be able to use whatever phone I want, gotten from anywhere, without needing to buy it through my carrier, and which I can keep updated without needing to crack it.
That means I refuse to use a carrier with a sufficiently proprietary network technology that enables them to be assholes about devices. This excludes Verizon and Sprint right off the bat.
So my only real options are AT&T or T-Mobile. Since T-Mobile has gone out of their way to be the least-jerk'ish mobile carrier in the US, while AT&T generally hasn't, I've basically stuck with T-Mobile. Sure, their coverage may not always be the best, but it does keep improving. And if I ever actually want to travel, I don't need to worry about having to shut off my phone to not be totally gouged on the bill.