T.O.D.
That is a tough one.  I guess my top five would be:
SLEUTH
THE MAN WHO CAME TO DINNER
THE LION IN WINTER
WHO'S AFRAID OF VIRGINIA WOOLF
THE DARK AT THE TOP OF THE STAIRS
I have done the first three and always hoped I could do the other two  Long past those roles now.  Among other problems.  I think my favorite of the five is DARK AT THE TOP OF THE STAIRS.  Maybe because I never got to do the show.  It is just such a beautiful touch of Americana.  It is DEATH OF A SALESMAN without being quite such a downer.  And the movie is so perfectly cast.  Robert Preston, Dorothy McGuire, Angela Lansbury, Eve Arden, and Shirley Knight, before she was a Hopkins.