LaBute is a pretty awful playwright who doesn't deserve to be anywhere near this thread.
Ibsen is essential but published his last major work in 1899. Miller wrote three great plays (Salesman, Bridge, and Sons - in that order) and it's true as Orson Welles said that you only need one, but I still don't think he reaches the heights of the others listed. So I'll throw my support behind Chekhov, Strindberg, Shaw, Beckett, Williams, Albee, Pinter, and add Stoppard as well. Have to become more familiar with Wilson, Hellman, Friel and Hansberry - though I did see earlier this year the Goodman's stunning production of The Sign in Sidney Brustein's Window, and I'm tempted to add the latter to my list on the basis of that play alone.