Tom Stoppard is a very gifted writer ("The Real Inspector Hound" is one of my all-time favorites), but sometimes I think he could really, really use an editor. His plays are often very idea-driven rather than character- or plot-driven, so they can easily become very static. Which is a problem when the play is around 3 hours long.
The Trevor Nunn version of Rock 'n' Roll was DEADLY.
I'll add another vote for Copenhagen. I saw the national tour at a very sparsely attended performance. My friend left at intermission. After a while, the people behind me just started having a social conversation. Usually, people talking during the show is a pet peeve, but I didn't hush them because what they were saying was more interesting than what they were saying on stage (although I did eventually move closer).
"What was the name of that cheese that I like?"
"you can't run away forever...but there's nothing wrong with getting a good head start"
"well I hope and I pray, that maybe someday, you'll walk in the room with my heart"