I have Verizon. I almost never have service problems. Although, it's more expensive than the others.
I had AT&T when they had exclusivity on the first gen iPhone. Shit service and shit customer service.
And Sprint is the worst of all of them.
Heard good things about T-Mobile in Vegas. May switch to them at some point since Verizon's international service gets expensive if you have overages on their limited monthly plans. Verizon doesn't have great international options if you're using your phone a lot offshore.