As a Native American who's been abroad, I can say that the best thing about the United States (because although Canada and Mexico are part of "America" too I assume we are only talking about one portion of this continent) is that it really is the "land of opportunity."

Canada is better if you are old, sick, disabled, or tired and want to do nothing and live off the government! (Be it the Government of Canada or Indian Affairs). The USA is better if you want a JOB. The pace of life in Canada is absolutely LETHARGIC. Even McJobs like cleaning toilets and mopping floors are few and far between and take forever to hear back from!

Mexico would be better if you like really hot weather all the time and can live entirely in Spanish.