Luigi Pirandello, Six Characters in Search of an Author
Friedrich Durrenmatt, The Visit
NOEL COWARD!!! (Blithe Spirit, Hay Fever, Private Lives, Design for Living, Present Laughter)
I have to second Thornton Wilder's The Skin of Our Teeth and Eugene O'Neill's Long Day's Journey Into Night.
Also, some modern 'classics':
Proof, by David Auburn
Doubt, by John Patrick Shanley
Last Night of Ballyhoo, by Alfred Uhry
The Last Days of Judas Iscariot, by Stephen Adly Guirgis
"Art is always in crisis: you must work fast to write in the breath on the window."
-Edward Bond