That's a tough one, because the major and second tier providers offer fews options, and there are so many trade offs.
Take T-Mobile, for example. Great customer service, widespread coverage but generally lower signal levels. That's clearly an issue for some users in some places. They use GSM technology and allow you to insert your SIM [subscriber information module] into virtually any unlocked mobile handset or smartphone compatible with their network. Great idea! But, realistically, unlocked handsets are typically not carrier subsidized, and therefore can be very, very expensive compared to those $ 49.00 full featured units offered by most carriers. That's offset to some degree by very low cost data plans, generous plans and bundling deals for HotSpot accounts.
Or, Verizon. Widespread network, strong signals in most locations, and innovative technology, including high-speed 3G data service. Coupled, unfortunately, with ridiculous policies, high data link prices and features like DUN-incompatible V CAST. They offer innovative smartphones like the Treo 700w, but few [and maybe soon, no] iSync compatible mobile handsets. Instruments are carrier-locked, and generally are missing the Bluetooth OBEX profile, in order to protect the revenue they derive from the cash generator they call Get It Now! Nice touch. Reason alone to abandon them, from my point of view. But, then there's that strong signal, the high-speed network, innovative options…
Cingular [soon to once again adopt the moniker ATT Wireless] is somewhere in between, but like Verizon and Sprint, uses primarily carrier-locked mobile handsets. They will likely debut the new Palm OS-based Treo 700p replacement for the Treo 650 next month. Of the four major providers, they probably offer the greatest number of iSync compatible handsets at the moment, but they too are far from perfect.
I love T-Mobile for the freedom and service they offer, but I chose them years ago as a carrier primarily for their early policies like Friends & Family and innovative plans they offered long before others did. I enjoy their very low cost data service, but it doesn't begin to match the high speed offerings from Cingular, Sprint and Verizon.
So, it's all a set of trade offs and compromises. And it's likely to remain so, until US carriers change their business models significantly and remake themselves in the image of the major providers in Europe and Asia.