In terms of coverage, I don't think you can beat Verizon Wireless.
However, and this is really important for more rural areas, make sure you get a Tri-Mode phone, and not an All Digital phone. There are rare times, but times none the less, that I'll get only an analog signal, and its better than nothing at all.
I have friends with all different carriers, and I can count on one hand the number of times they've had a signal and I haven't.
Good Luck.
