Just about any good brand of pelleted feed which has has a 14% protein-6% fat content, along with a decent vitamin/mineral package blended in will do fine.
As rule of thumb.... just remember that protein builds muscle and fat builds fat.
But just feeding protien won't do much. Along with a proper diet, ya gotta also do something to stimulate muscle growth. So you'll also need a regular exercise program to go along with that high-dollar feed. Otherwise, you'll just end up with a fat horse.