When selecting beef for stir-frying, look for cuts that are well-marbled, as this will help keep the meat moist and tender during cooking. Eg: rump, sirloin, flank, rib eye, and skirt. Remember, fat equals flavor and tenderness. Tougher cuts or those with less fat, are less suitable for stir-frying. eg Shanks, rounds, shoulders, brisket, and chuck are unsuitable for stir-frying. Unless you are using really expensive cuts like wagyu, Kobe sirloin, or tenderloin, it is a great idea to tenderize the beef before frying.