If you want the juiciest, most flavourful steak you can cook, look for plenty of fat throughout the meat. Serious Eat's guide to grilling steak says marbling is a more important quality to look for than just the grade of the beef.
Photo by Ernesto Andrade.
Why is this? Well, that marbling is responsible for both moisture and that beefy flavour:
As well-marbled meat cooks, the fat will slowly melt, adding juiciness built right into the meat. Non-marbled meat might have plenty of fat on the exterior, but it doesn't enhance the steak in the same way. Sort of like the difference between drinking a glass of chocolate milk or drinking the milk then shooting the chocolate syrup.
Flavorwise, almost all of the compounds our tongues sense that give us the thought "ooh, that's beefy" are found in the fat. In fact, if you take the fat out of a piece of beef and replace it with lamb fat, it'll taste like lamb. Want chicken-flavored beef? Cook lean beef in chicken fat.
J. Kenji López-Alt also offers 10 other tips for cooking a perfect steak, particularly on the grill. Many of the tips we've mentioned before, such as turning the meat often for more even cooking and to reduce cooking time, salting your steak well in advance, and letting that steak rest.
The Food Lab: How to Grill a Steak, a Complete Guide [Serious Eats]