Verizon has the absolute best coverage by FAR. I've had Verizon, and only switched to AT&T because of the iPhone. However, Verizon has done a pretty good job with catching up with the technology of the iPhone, if you're at all concerned with that.
If you want to make sure you have service in a ton of places, get Verizon. AT&T coverage sucks, and Sprint is a pain in the butt to deal with (my dad has a sprint phone and hates it).