I'm down with Jeffrey Steingarten, Vogue's food writer, who says that the carapace of super-fatty beef that curves around the top of most rib-eye cuts is the best there is (though a case can be made for the tender smaller side of a Porterhouse or T-bone).
South American beef, which is grass-fed, is leaner and contains more omega-3 fatty acids than grain-fed American beef -- which explains, in part, how South Americans can eat such gargantuan quantities of it in one sitting, and then do it again the next day.