AT&T.
Not Cingular, though. I liked Cingular. But ever since the switch to AT&T I've gotten nothing but terrible customer service. Every time I have an issue with my phone, either customer service is closed (they keep very strange hours, IMO), I sit on hold for an hour or more, or I feel like I'm talking to a wall that refuses to understand what I'm trying to say to them. Cingular had excellent customer service and that almost made up for the terrible cell service.
Also, the rate of dropped calls has increased since the switch. I've been told that's because they have added additional towers and now I'm more likely to be on an AT&T tower than on a 'partnership' tower, so even if the AT&T tower is farther away my phone will still attach to it instead of the closer, more powerful partnership tower. How much of that is true is debatable. I'd say about one out of 20 calls I make or receive get dropped within the first 2 minutes. Many do not even go through when I dial them. Last week I was having a problem where I could not dial a friend's phone number - got a bizarre tone and wouldn't connect. Only when dialing her cell phone, no other numbers, and she could call me and we could text back and forth without any issue at all. Three days later, the issue mysteriously resolved itself. Anytime I'm driving (which is often, I commute about 90 miles round trip for school three times a week and my bluetooth headset is my best friend, and my husband's family lives about 600 miles away) the call will drop about every 30 to 40 miles. Tech said this was because I'm transferring towers, I question why no other carrier I've used had this problem.
And in that same vein, I hate my Motorola phone. You'd think I'd have learned by now that Motorolas are awful, but I came by my current one for free right around the time my old Motorola was breathing it's last breaths. Between myself and my husband, we went through seven Motorola V-series (started with v400s, ended at last with v557s) within 2 years, only 2 of them were broken through any fault of our own. I had the screens go out on three, he had the flip hinge break once (after about 3 months of use, was deemed a warranty failure), screen go out once, broke one during a car accident (which was actually my phone, that he had with him that night), and dropped one while shaving. Now we're both on Motorola Razrs, the V3i version (which there are no faceplace covers available for that actually fit them - I can only find ones made for the V3c, which are slightly different). My brother gave them to us because they were the free phones when he opened his AT&T account and he finally got his and his wife's phones from his old carrier unlocked and transferred. In the single month he had them, he had one overheat while in his pocket. Mine is always giving me problems. Screen flickering, static, 'white screen of death,' calls not coming through, messages not coming through, etc. My husband has similar problems with his.
We're waiting (im)patiently for the LG Voyager to come down in price and then switching to Verizon.
*steps down from soapbox*