Canada

We already have a major free trade agreement with the EU, and we're not burdened by their regulations on most resources that we want to export to them anyways.

This is different from those people who are talking about actually joining the EU. We're just shipping our raw resources (or preferably refined resources).

And frankly speaking, a lot of the US's growth comes from the fact that they've been taking advantage of their legacy infrastructure like 60 year old power plants, and the fact that we sell them oil and minerals at costs much lower than the world average. We redirect a ton of that to the EU and other countries, and the US's growth with drastically slow down while giving us huge growth as the profit margin of our products will rise substantially.

EU might not be as rich as the US, but it is still quite rich, and isn't about to collapse under its own weight like China. As economies go, they are literally the next best thing after the US, and their reliability makes them far more attractive partners for the next half century.