In The Independent Johann Hari asks Is Tom Stoppard's Arcadia the greatest play of our age ? -- and argues that it most definitely is.
I'd find it hard to disagree. Arcadia is the only drama under review at the complete review that rates an A+, and has always been one of my favorites. My only hesitation about anointing it best of the times is that while I've seen it performed three times I've never seen a performance that really lived up to the play. They've gotten the staging (those switches in time) down well, but the performers haven't ever been completely up to it -- specifically the hard to cast Thomasina. None of the three Thomasinas I've seen have been anywhere near adequate (largely, I suspect, because I have never seen the role entrusted to anyone who is anywhere near Thomasina's age).
So while in the abstract Arcadia does strike and haunt me as one the greatest modern plays, I'm not sure about it as a stage-play. But then I prefer to read dramas rather than watch them, anyway -- I have a vivid enough imagination to play them out in my mind's eye, and Thomasina is sublime every time in there.