What kind of roast are best in crock pot?

Chuck, brisket, shoulder, or other cuts that have some good fat content, but are tough cooked in any other way. The tough collagen in between the muscle fibers literally "melt" over time (slow braising) into the delicious texture ociated with these flavorful meats. "Stew Meat" and other "unspecific" labels often give you cuts from the "round" too lean for this method.