Sprint for me. Although it may not be the best in some areas right now, they are buying US Cellular which will improve the network a ton. Also, when the full LTE rollout finishes up in my area this month, I will have unlimited LTE. This is going to be great for GPS, Music Streaming, YouTube, ESPN, and other heavy data apps.
If you are in an area where Sprint has improved, is getting, or has LTE, it's the jackpot.
For me, there is no question. T-Mobile is my only choice. AT&T and Verizon can't seem to ever get me coverage at my house. My data speed tests would be edge speeds, hardly enough to spend $85-100 or whatever a plan costs through them these days.
I have speeds up to 41Mbps on LTE at my house on T-Mobile. They also have solid coverage throughout my neighborhood and where I work.