From the responses thus far, I gather we're talking straight plays? Good enough.
Top of the list, hands down: THE MADWOMAN OF CHAILLOT by Giraudoux. Almost anything by Giraudoux, actually, but in the original French, not those lame (and inaccurate) Maurice Valency translations.
HOTEL PARADISO by Feydeau. Well, again, almost anything by Feydeau because they're such wonders of construction. And just when you think it cant get any more bizarre, he sends it right over the top.
Anything by Aristophanes, but special mention to LYSISTRATA and THE BIRDS.
EDWARD II. No reason needed.
SKIN OF OUR TEETH. I would seriously *kill* to design that play.
HOSANNA by Michel Tremblay. Again, no reason needed -- it's just a wonderful play, period.
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