This is a tough question and I'm not going to vote because of the possibility that this might mislead somebody. The truth is that the best provider is a very vague question. I'm assuming you mean a combination of coverage, prices, phones, and customer service. But it's up to you if any one of these factors is more important than another. It also highly depends on where you live because every single provider has areas where they dominate and have the best coverage as well as areas they need a lot of improvements.
Enough with the speech, this is how I normally advise people to choose a wireless provider. Ask people around you who they use and if they're happy with their experience. If you are going to use an online forum, at least give a location. If you are in an area where you have a smaller provider, Alltel or USCC, go with them. The smaller companies pay more attention to the customer, have better rates, plans, and give you more for your buck. If you travel in an area not natively covered by Alltel or USCC, you will roam for free on the larger providers. You'll get all the benefits of a national provider while not paying the higher prices. Next I would give the edge to T-Mobile. T-Mobile lacks a lot of more rural coverage, but they have great customer service and are actively building and increasing their coverage area.
I live in a market with all the big boys, VZW, Cingular, T-Mobile, Sprint, Nextel, Alltel, and NTelos. As you can see, I went with Alltel and T-Mobile mainly for the reasons I detailed above.
I would next give the edge to Sprint. Sprint offers more in a plan than VZW and will allow you to roam on Alltel and VZW anywhere Sprint doesn't have coverage. Sprint also offers nights at 7 so their plans are a little better than Cingular/Verizon, who I would rate as last. They are the largest cellular providers and have the highest prices. They often advertise their point of reliability and fewest dropped calls and live off their reputation which is mainly created from advertising. It may be that one of them provides the best coverage in your area, but if there is another good option, it may be best to go that way rather than just thinking that other providers won't be as reliable because they don't pound the message of reliability with their commercials. But they generally do carry the latest and greatest phones because they're able to pay for the exclusive contracts to carry them.
Of course there are a ton of other factors to consider... who's free mobile-to-mobile calling will benefit you most? Do you call many landlines or have friends on many different networks, My Circle might be a good option. Maybe you live in a deadspot where one provider provides coverage where others don't. if you don't do much calling, a pre-pay plan or a MVNO would probably be your best bet.
Most markets don't have coverage from every provider, so the first step would be to go around to all the websites and shop for service. Some websites probably won't let you after you enter your zip code to browse for plans, that's always a good first step.
Although I kind of danced around the question of who is the best, maybe this will give you a few things to think about and consider for the area in which you live and your calling habits. Good luck in choosing the provider that best suits your needs.