AT&T, but Verizon might be in my future (this summer)
I've been with Sprint for many years. (I switched to get the Palm Pre, if anyone remembers those!) The iPhone 5S was such a huge upgrade for me, as it was the first with LTE speeds. Now, really happy with Sprint's LTE speed and coverage in the major cities.
VZW but not for long, Data is slow and the bill is high
I really have been thinking about it.
Activate a line first to try it. That's what I did. Once I knew T-Mobile worked for me, then I ported mine and wife's numbers and Att is now history.