GoogleFi uses Tmobile, Sprint, and US Cellular, plus has the same cost data roaming as in the US ($10/gig) worldwide. They also have a cap on price if you use a lot of data. Also, very low overseas calling costs.
Get a google branded phone (sold on their web site) for best results. It has to be Android. iPhones work, but won't seamlessly switch networks.
Ting is another good bet, $6/mo for the number, then they charge you for your actual usage.A light user will cost $15 or less per mo.
Comcast (Xfinity) gives your free service if you have internet through them. You pay for data.
iPhones are more compatibite with multiple carriers, but older ones do not have the latest frequency bands. Swappa.com is good used source.