The most tender part of a cow is near the butt end of the cow since that part of the cow has to do less work than other parts of the cow.
The Sirloin which is the primal cut and close to the butt end of the cow is the most tender meat part of a cow.
Some other cuts of meat from a cow that are pretty tender but not the most tender are the Hangar, Flat Iron, Rib Eye, Tri Tip and the New York Strip.