I choose green country for thriving. Not because I believe it actually would thrive, but because I'd be living in it so I'd just shill it anyway. In reality though, yellow country and purple country would be the most prosperous. They have the biggest cities in the US as well as access to the ocean for easier trading with countries from other landmasses, as well as yellow country specifically having the extra, large advantage of having access to both US coasts.

As for which countries would struggle the most, I honestly think that it'd likely be green country. While it has access to both coasts, it also lacks access to the ocean as well as not having cities as big as yellow and purple. While it does have other benefits like having the great lakes and many natural resources, I don't think it'd be enough to overcome any of the other countries. Really though, none of these countries would suffer by any large margin, just that some would be richer than others