The best part of beef to use for goulash is a chuck.  Chuck is cut from the shoulder that has good marbling, which gives it excellent flavor and makes it tender when cooked slowly.  Chuck can also stand up to long cooking times, making it ideal for simmering in a stew.  If you're using cubes of beef for your goulash, look for cuts labeled "stew meat" or "beef cubed for stew", as these are usually chuck cuts that have been pre-cubed by the butcher.  You could also purchase a boneless chuck roast and cube it yourself at home.  Whichever option you choose, be sure to trim any excess fat before adding the beef to your goulash!