Nextel has nearly 100% coverage in WDW. There are very few spots that it gets iffy inside very few of the buildings.
WDW uses nextel Walkie Talkie for communications property wide, so thats why Nextel is rock solid.
US Cellular is also decent. I couldnt get coverage in a room in the contemporary, but just outside the room in the hall near the windows was fine. Inside rides, USCC doesn't work, nor in some of the shops or buildings, but coverage was still pretty good considering USCC doesn't really cover the area ( it's a midwest service, so it's all roaming, which is fine as long as you have Nationwide on your plan. If you don't, use seldom, trust me! )