I like stuff to just plain work. I have been with AT&T since they were bought by Cingular and back again. They haven't ever failed me.
Buy your own phone, unlocked, and NOT from a carrier (I just bought the Motorola Moto X direct from Motorola). You can get on w/o a contract and change as you see fit from month to month. They now rollover data. We use the 10 GB for the 2 of us, and it's been fine.
Make sure you tell whatever carrier you are with which company you work for. There is likely a discount. I get 22% from a company discount.