I am currently with Sprint. In my opinion, the plans are cheaper and efficient. With AT&T, they are expensive, but you have rollover minutes and the signal is not faulty as Sprint, although I haven't had a dropped called or anything signal related problems under Sprint service.
neither, verizon is the best. but if you had to pick one then at&t. my friend got a sprint phone not that long ago and all of a sudden her phone screen doesn't work anymore. she cant text or see texts or know who she calling. she now has to get another phone. i have had at&t and the only bad part is that it is hard to hear the person on the other line.