I don't think there is going to be one best carrier. It's going to depend on each person as their use of the cellphone/data service is going to vary. I personally have T-Mobile and I've had it for the last 5 years and only added the BB data service on 2 years ago. I live in the San Gabriel area and coverage is pretty good but I work in Century City and alot of times when I'm in the buildings I don't get good reception. Other people on AT&T and Verzion get better service in the buildings. Am I going to switch...most likely not. I'm not on the phone that much and for me to switch to another carrier I would be paying almost 50 bucks more a month to get the same service as I currently have.
I pretty much know where all the dead spots are in my travel route and if I need to make a call I usually just wait until I pass that dead area then make the call. Slight inconvenience but it doesn't happen alot and I'm sure there are dead spots with all the carriers too. Just my 2 cents.