Cellco partnership has a lot of native coverage, with some big holes, but they have the biggest native network. AT&T has a lot more roaming, but they also have a large native network. There's always weird exceptions like CellCo roaming on CellularONE of northeast PA because Verizon doesn't own PCS or CLR there. That problem will be history soon, since CellCo can build 700mhz LTE in any area that has high roaming traffic, so only voice and text have to roam.
Sprint's native coverage is pathetic. There's holes in their coverage in CONNETICUT. We're a tiny state with tons of people. In New Hampshire, their coverage ENDS at the bottom of the big lake, which has a gazillion tourists, so native Sprint is pretty useless if you need to be connected while traveling or vacationing. Also, on contract Sprint, the only reason you have such good coverage in rural areas is because you're using CellCo towers. Sprint only builds a skeletal network when you're not in urban/suburban areas. Even T-Mobile's EDGE network is bigger! Sprint needs to get their stuff together, and build more towers, and consolidate their existing CDMA/iDen coverage. The thing is, their network is not set up for prepaid, it's setup for post paid, where it's cheaper in many places for them to pay CellCo to roam than it is to build native coverage. Basically, there's no free ride in wireless, and you get what you pay for.