Whose face is on the US 500 dollar bil?

President William McKinley. There's a picture at the Related Link, below

The last official printing of a $500 bill by the US was the 1934 series. Others preceded McKinley. It would depend on the issue date of the note. Distribution of bills higher than $100 was halted by presidential order in July 1969. They've never been withdrawn or demonetized but are effectively no longer in circulation.