If you can afford it, the standing rib is your best bet. Inside round works well also, but requires slow and moist cooking methods to give you something chewable. Be sure to choose at least USDA Choice if in the U.S. If you have a local meat processor that hanldes local beef, you can get Prime grade for a good price. Otherwise, it's hard to come by.
And remember, especially if you use the round, cut thin and against the grain.
In any case, enjoy.
Seeeeeeya; Goodweed of the North