AT&T. My first carrier was Cingular, and when they bought AT&T I was a happy customer. I left AT&T late 2009 when I got pissed about their rapidly degrading coverage at my house and they would do nothing to help rectify it (wanting to charge an arm and a leg for a microcell) and went to Sprint for a year because of their unlimited data plans, only to find out they could offer unlimited data because their network was too slow to use in any meaningful way. Still, I was generally okay with Sprint until my wife's service started having really weird issues, and instead of trying to fix it they kept giving us the run-around and insisting she simply change her phone number. I paid the ETF and went to Verizon for about 24 hours, until they asked me to find another carrier because they were never going to offer coverage where I lived.
I went back to AT&T that day and haven't looked back since. The grass is always greener...