Like aforementioned, check the coverage in your area.
Typically what I have found is as follows:
Speed: Verizon & AT&T top the list
Cost: TMobile is the cheapest
Cost/Speed Combo: Sprint has a good cost/benefit balance
If you aren't too concerned about price, then I would avoid T-Mobile. In my experience their data is the slowest and runs, at best, on Edge. They have no Blackberries running on their 3G network. Sprint has a simply everything plan, they have nice devices and their speed is decent.
AT&T and Verizon are pretty comperable when it comes to price and coverage. AT&T is known for poor customer service and Verizon has complaints about the option their blackberries provide (like the lack of wifi, for example.)
It really all comes down to how much you are comfortable paying and what you want to get out of your blackberry.
RETIRED: 7230, 7290, 8700G, 8320, 8900. 9000 BOLD