Which cellular service providers have the best coverage?

It is by far Verizon Wireless. It has the biggest coverage in the United States. The 3G Verizon has has the biggest coverage then any other company. Just because you have the 3G iphone does not mean your getting 3G speeds just check out the coverage map. Verizon service and Data just flat out beats any other company. From the cell service to the internet air cards Verizon is a head if it's class.

I know Verizon has the best coverage but sprint has the third best and it is the cheapest

(Modification to previous answer) While Verizon Wireless has the best 3G coverage in the United States, AT&T remains to have the most overall coverage/