Of those, the X-Files and nBSG probably have to rank as the worst final seasons for a show, primarily because TPTB had no fucking clue how to close all their loose ends.
I wonder how others would rank them...
I never watched X-Files or Heroes, but I enjoyed the final runs of nBSG and Lost immensely. Both stayed true to the mythos that were set up in earlier seasons. Many people disagree with that, but the clues were there all along.