Shanks Shoulder Round Rump
Marbling, are the fat patterns in cuts of meat, although marbling is not wanted in lean cuts. This fat in the meat is desirable for the slow cooker. So skip the expensive cuts. My experience is that they become dry in texture even when surrounded by moisture from the liquid added. Fat is beneficial in the slow cooker. You might say very necessary.
Stew cuts, chuck roasts, all work really well and give you a good texture and flavor for your recipe. If you find a round steak with little marbling at a good price, pass it by. Its just a bad choice.