The typical cut of beef is the chuck roast. This comes from the neck and shoulder area of a cow. Some grocers will even lable this cut as a Pot Roast while others may call it a chuck or chuck eye. Other cut options would be bottom or top round, or brisket. You want a meat that is thick with a lot of cartilage and visibale marbling. As it cooks, this marbling or fat, will melt away within the meat making it fork tender and easy to shred. More lean cuts such as a shoulder roast would require slicing vs shredding - different texture.