It kinda depends what you need.
Simple voice coverage will be dependant on where you live - ask your friends. In my experience of extensive travelling throughout the country, Verizon have the most consistent coverage. That said, I've only tried T-mobile, Verizon and AT&T.
If you're planning on using data services avoid T-mobile. Their 3G rollout is way behind Sprint, Verizon and AT&T.
If you ever plan on using the phone overseas, avoid Sprint and Verizon - their networks are based on the North America only CDMA standard.