I normally try to make notes of this stuff when I am in the parks and what I have found is that the BEST coverage is from Sprint/Nextel. This is due to the contractual obligation that Sprint/Nextel has to Disney. 10 years ago Nextel purchased WDW's radio frequencies so that Nextel could expand it's footprint in Central Florida. As part of that agreement, Nextel would provide Push-to-talk and cellular service to the WDW resort. This included signal enchantments in all areas of WDW. Most of the towers are hidden as trees along with Bi-Directional Amplifiers (BDA's) in most internal areas of the parks (especially the utilidors).
ATT has "OK" coverage. There is a big dead spot near France in the World Showcase and 3G coverage is sporadic in the resorts. I have not had a verizon phone in the parks in years so their coverage might have improved.
Unless WDW has signed an agreement with other providers to allow them to share tower space somewhere on property, most coverage is going to be from the "outside, looking in."