Hanger steak (or onglet), especially if you can get USDA Prime. The flavor is intense (verging on gamey but in a completely pleasant way). Cooked medium rare it's the perfect complement to frites. Leftovers make a great salad. Seared quickly in butter, cooked just to rare, and then moistened with a dash of vodka and left to cool it makes the most sublime roast beef.
Interestingly, the cut seems to be more common on the East Coast. When I went to visit my brother-in-law in Arlington, VA earlier this summer it was all over the place. For some reason, though, the VA beef was much less distinctive than what I buy in California. Even the vaunted Organic Butcher of Maclean VA failed to deliver the goods.
Ever since Mad Cow added its own piquant frisson to beef eating I've been unwilling to indulge in supermarket beef. It's really worth it to find a good butcher with access to both dry- as well as wet-aged beef in a variety of grades up to and including USDA Prime.