Sprint because they're the company I've always been with and because they were cheaper than the alternatives. Service is good where I live but terrible in my office, which seems like an okay balance.
Sprint because a friend works there and we're on his "friends and family" plan so it's SO much cheaper. We used to be w/ Veriizon and we miss it. We always got better coverage with Verizon. But the $$ savings makes it worth it.
AT&T. I've been with them since I turned 17 and got my first crappy Nokia cell phone. Well, except for about a year or two in my early 20s when I had one of those chirping Nextel phones.
I get an 18% discount on my bill and 25% discount on phone and accessories, all through work.
I have strong service from central PA, through Amish paradise, and all the way to south Jersey near the coast. Verizon and AT&T are the most common in this area. Sprint and T-Mobile have crappy service around here.