Who is the legendary cowboy last name bill?

There are a couple of possibilities.

If you mean "legendary" literally, as in "he was a legend and not a real person", then it's probably Pecos Bill.

If you mean "a real person about whom legends formed", it's probably Buffalo Bill.

It's worth noting that in neither of these cases is "Bill" actually their last name. Buffalo Bill's last name was Cody; I don't think Pecos Bill had a last name given. The "first" name in both cases is a nickname: "Buffalo" Bill Cody and "Pecos" Bill Nolastname.