That is an unanswerable question. Both Sprint/Nextel & Verizon reach about 80% of the US population but not the same 80%. And Cingular has the physically largest network of the big three, but not significantly larger than the others. At any gven place in the country you will have service from 1, 2 or maybe even all three of these carriers, but there are also places that have no service at all from any of them. And in some places one carrier will have a strong signal and the other a weak one, just because their towers are located differently, e.g. there is a hill or building between you and one of the towers. Move a mile or two away and the situation might well be reversed. Many of us carry two phones from different carriers to try to maximize accessability.
If there are places that you will frequent more than others, make sure the carrier you choose has good coverage there. You can check their coverage maps, but they are broad brush and don't account for local dead spots. Its better to ask someone who frequents the area how their reception is and who they use.
Bottom line is you can pick whichever one seems most attractive to you. Verizon & Sprint are probably better choices (right now, anyway) if you want to use the cell phone to get online as well as making voice calls. They have the best data services right now.