Cube steak, the standard choice for chicken-fried steak, isn't a particular cut. Rather, todao says, it's meat that's been put through a tenderizer, "which makes countless little indentations in its surface." Uncle Bob points out that a single cube steak might even consist of multiple smaller trimmings run through the tenderizer, folded, and run through again.
For chicken-fried steak, todao recommends a nice piece of top or bottom round measuring a half-inch thick. Ask the butcher to remove the sinew (if present), and then to take the steak down to about a quarter-inch thickness in the meat tenderizer. Finally, let it rest for 15 minutes after dredging in flour, egg, and flour again. Fry it golden brown in a hot pan.
Discuss: Cube steak