General consensus these days is that T-Mobile is one of the best contract plans for the price. Coverage depends on your area. If T-mobile isn't good in your area, consider trying something like Straight Talk (if you can still find an ATT based SIM to activate). Roughly $45 a month "unlimited" everything (soft data cap at ~2GB). Can run on ATT or T-mobile network. You need to bring your own phone, though.
VoIP solutions (like Google voice + supporting app) are growing in popularity, but if voice quality is important to you and/or if you use a lot of voice, might as well go a dedicated plan.
Edit: I'm biased towards GSM providers since I like the freedom of swapping SIMs into new devices.
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