[Ditter]

Which one is best over all?

[eno_]

Depends on what you want:

Verizon for the best service in the U.S.
Nextel for the second-best, and/or if you like push-to-talk, and you don't care if the phones are a bit large and ugly.
AT&T if you want GSM and cute Nokia and Siemens handsets, and for portability to Europe.
If you travel to Europe a whole lot, and you want a plan that covers Europe and the U.S., and you don't mind really abysmal coverage in the U.S., T-Mobile.

I don't pay for my own phone, so I have no idea about pricing.

But the above are only true nationwide. If you want only service in and around your hometown, you have lots of choices, and criteria depends more on local service.