Google's Project Fi combines Sprint, T-Mobile, and U.S. Cellular into a into a virtual network with better coverage than any one of the three offers individually.
You'll pay $20 for the first line, $15 for each additional phone line, and then buy only the data you need. If you go over on data usage they simply add it to your bill; if you're UNDER your data usage they refund you that portion of your data. There's no contract, so you can cancel any time.
If you travel a lot and want to use your phone outside of the United States, this could be the plan for you. Project Fi lets you use your phones data normally (read: with no insane roaming fees) in 135 different countries. You can also make phone calls over Wi-Fi normally, or place calls on a cellular network for 20 cents per minute. Texts aren't any additional cost whether on Wi-Fi or a cell network.
In short, unless you're making a lot of calls outside of Wi-Fi, your phone will work normally at no extra charge at almost any country you want to travel to.
It has fast and robot-free customer service that you can access via your phone, the web, etc.