It would be a good feature if the map tooltip also included "Rank: {rank here}". Since we already have that information, it would be a good addition for users who want to see how countries fare with respect to other countries.
This would also clear out any ambiguities caused by color differentiating only a wee bit. It would be a good add on for accessibility too as color-blind people can still know which country's ranking how, without needing to have to go through minute variation of the green color. Screen readers can also benefit from this IMO.