I'll third Voyage and second Candida. They sound interesting and similar to Hamlet53, they are the two I haven't seen before.
Nominations so far:
Glengarry Glen Ross by David Mamet 
The Glass Menagerie by Tennessee Williams 
Voyage by Tom Stoppard 
Candida by George Bernard Shaw 
Rosencrantz And Guildenstern Are Dead by Tom Stoppard 
There is about a day and a half left for nominations.