Brand loyalty to a cell phone company is probably stupid, but I do kinda endorse Google Fi–Google’s mobile service. It charges a flat $10 per gigabyte of data and, if you don’t use all you’ve allotted for a particular month, you get a credit for the next month.
It’s not the cheapest plan I’ve had (on a per-gig basis), but it’s the most transparent. And its data plan works the same abroad as it does in the US.
The Google Fi’s biggest drawback is that it only works with a limited number of phones (sorry, no Apple devices).