Flat cut is the best and most widely available option. It's leaner and the best for slicing. Corned beef usually comes with a seasoning packet containing pickling spice ingredients. But you can also ...

Corned beef and roast ... of any number of large cuts of lean beef, such as rump, sirloin, or tenderloin, which are seasoned liberally and roasted until tender and flavorful. Common herbs and spices ...