How about oxtail (which is beef)?
This recipe on food52 from MyCommunalTable looks good:
or this one from crispywaffle:
They don't use a crockpot, but their method is braising low and slow for several hours, so you could do it in the crockpot.
Beef ribs work well. I have no cut & paste recipes to add but Cooks Illustrated has great ones - also check Williams-Sonoma recipes, and the web.
Shanks for such a great question! Beef, lamb and pork shanks love the low slow crockpot. I do neck and shoulder cuts like blade steak, "7"steak and even bone marrow bones work well in the crockpot.
and for a real treat if you can find goat meat and pig tails for Jamacan stews and curries.
Bottom of the round. Brown first and put in crockpot with small amount of beef broth, onion and optional dried bay leaf.