Waiting For Godot
Why? Well for a no of reasons, but the fact is that Waiting For Godot is a play that relies heavily on dialogue and in the hands of lesser actors it can easily drag, but Sir Ian McKellen and Patrick Stewart made much of their parts and managed to get across the individual characteristics of Vladimir and Estrogen which is, as far as I can see, that Vladimir is an optimist against the odds and Estrogen is a pessimist with nihilistic and suicidal tendencies.
Its also a play which needs to be read on two levels. To go into the various existential interpretations is perhaps not the thing to do here as there are many to list, but in spite of Beckett's assertions that there was nothing beyond what he wrote, I have to say that there does seem to be a hidden agenda here and that nothing is coincidental as Beckett was too canny a writer. But then it could be that the agenda is simply to make the audience think.
All in all definetly worth seeing if you get the chance.