Choosing your meat is at least as important as how you cook it (of course, you can ruin the best meat in the world just as you can't really rescue a truly inferior piece of meat). Extremes aside, I'm a big advocate of the under-appreciated cuts like skirt, hangar, flank, and flat-iron. Do I go after a gorgeous, dry-aged porterhouse, given the chance? You bet. But no matter what the cut is, I can never forget food politics, fat (marbling, that is), dry aging, and price when I'm deciding which steak to buy.
Food Politics
I prefer grass-fed, hormone-free meat from the more flavorful, slightly more exercised parts of the animal. That means I buy my meat on a regular basis at Whole Foods. In New York, I adore the naturally raised, grain-finished hangar steak from Jefferson Market (6th Avenue and 9th St.). And, of course, there's the industry standard, Niman Ranch. I have fewer choices when I shop at my grocery store but you may be more fortunate. Why eat natural, grass-fed steak? Because it has more flavor, it's healthier, and supporting ranchers who allow their cows to live what I call a "cow's life" (eating grass, walking around, enjoying the breeze) is the right thing to do. I like open pasture and I like the idea that the meat I'm eating is carefully cared for from start to finish.
Fat
Fat is a good thing. You want your steak to have plenty of fat - not only does it add flavor and richness, it gives the meat a lovely, buttery, texture. By fat I mean marbling, not the big hunks of solid white fat you trim off before cooking. High-end steaks from the Loin and Rib section have more fat and less gristle (they don't do as much work as say, the muscles that make up the Sirloin or Round). I adore steaks from the Rib and Loin sections but I lust after the super-flavorful, easy to cook steaks from the Plate and Flank. Why do I choose skirt steak over flank steak? Because skirt steak is a very fatty cut. Hangar, which I also like for its big beefy flavor, falls somewhere in between. How can you find a nicely marbled steak, whatever cut you go for? Look at the meat. It should have plenty of spidery white lines running through. That's fat and that's the meat you want. (It should also be a nice bing-cherry color and smell like nothing so much as clean and good.)
The grade of steak you buy is another way to find fattier, tastier steak. Prime steak is, of course, the top grade given by the USDA and it designates meat with the most marbling from animals in top condition. Then there's Select which is what most of us are buying and eating at home. Choice is the next grade down. You probably want to avoid it.
Dry-Aging
Dry-aged beef is a delicious treat. In part, the great steakhouses have more and more of a corner on this kind of meat because, unless you live in a big city or have a terrific butcher, it's harder and harder to find dry-aged meat -- never mind finding all-natural dry-aged steak. Dry aging is a process that takes between 30 and 45 days during which meat is allowed to rest and dry while its natural enzymes go to work, developing flavor and tenderizing the meat. This is all good once it reaches your fork. But dry-aging is expensive, so most meat these days is wet-aged, which isn't really aging at all. It simply involves prolonging the shelf-life by packing the meat in its own juices. None of the good stuff that happens when meat is dry-aged happens during wet-aging. Alas. Buy dry-aged meat if you can.
Price
Beef prices are higher than they've ever been and grass-fed beef is yet more expensive. That's why under-appreciated cuts like hangar steak, flank steak, flat-iron and skirt make so much sense. They run roughly half of what the classic cuts coming from the Loin or Rib will cost you.