Who is the last word given to in the American system of law the accused is innocent until proven guilty?

The last Word, so to speak, is given to the jury. If it decides a person is not guilty, there can be no questioning how it came to that conclusion. The person is free of that charge forever.

Another "last Word" is that in a criminal trial, the prosecution gets to make the final closing statement. In that respect , the prosecution has the last Word.