I have been with T-Mobile & Sprint in the past, and I find AT&T definitely surpasses the other companies. I had constantly had unresolved connectivity problems with Sprint. I had to literally stand in the middle of my road to get reception at my house! (I discovered my neighbor also had Sprint when we met in the middle of the road one evening!)
I didn't have any connection problems with T-Mobile, but they have extremely rude, unresponsive customer service reps. I had a problem with my billing & they were offensive, insinuating I was lying. Once I proved my case, their attitude did not change. The CSR's working in the stores are rude also. I guess it's just their company's nature.
AT&T has been wonderful. While they could improve on the wait time for a CSR, once you reach a rep, they are extremely open-minded & helpful. Pleasant customer service goes a long way! I have not had any reception or phone problems with them. I've been with them for about 5 years now.