I would say it's the United States.
In the late 80's Mexico ruled this region, but with the scandal leading to their ban from the 1990 World Cup, the Americans were able to gain entrance to the World Cup.
This, in part, led to our confidence in 1994, when we advanced to the second round and really should have done so much more.
In 1998, we were terrible, and were much weaker than Mexico.
However, with our amazing triumph in 2002 over Mexico, our biggest rivals, we've re-claimed the top spot as the best nation in this region.
I am sure the Mexicans will make another run at the top, but it's becoming more and more isolated between ourselves and Mexico and the rest of the region. Not Rangers and Celtic-esque, but there's still a distinct lack of class.