My complete thoughts on Modern Plays:
The History Boys was lame.
Totally disagree. One of the best plays of the noughties.
Doubt: A Parable was okay.
Proof wore a little thin.
Dinner With Friends was excellent.
Copenhagen was a bore.
As was W;t.
Closer just stinks.
I actually quite liked it. It's an example of in-yer-face theatre (very provocative, normally graphic/explicit). Not sure how it works on film.
I love the music of Rent.
Broken Glass wasn't as good as Miller's earlier work.
Three Tall Women was great like everything Albee does.
Tony Kushner is a great new talent and Angels in America shows real promise.
Death and the Maiden's ending didn't work for me.
I can't read or sit through any of Neil Simon's work.
Based on his efforts in The Piano Lesson and Fences I'd place August Wilson somewhere between Tennessee Williams and Arthur Miller.
The Heidi Chronicles was feminist drivel.
A Few Good Men had some memorable dialogue.
Driving Miss Daisy was enchanting.
M. Butterfly was nauseating.
I'm Not Rappaport stinks.
Dangerous Liasons was excellent.
Sunday in the Park With George was boring and didn't have any good music.
Glengary Glen Ross was stupendous, a must see. David Mamet knows dialogue.
'night Mother was depressing, but insightful and oddly touching.
A Soldier's Play is terrible.
Noises Off was slapstick and laugh out loud funny.
Agree. I love this play- always gets done in amateur companies and always a winner
Amadeus, wonderful, can't say enough good things.
Children of a Lesser God, sexy and intelligent.
The Elephant Man, man is a wolf to man.
A Chorus Line, glitz glamour, as good as Bob Fosse's stuff.
I thought the general consensus was that it was a bit lame
Equus, thought provoking.
That Championship Season, has it's moments.
Rosencrantz and Guildenstern are Dead, Stoppard proves himself a force to be reckoned with.
A Delicate Balance, almost as good as Who's Afraid of Virginia Woolf?
The Homecoming, didn't quite come off, but there is a there there. Pinter's The Dumwaiter struck me as better.
The Dumb Waiter was boring and based on a boring conceit. Betrayal is great though
The Lion in Winter is probably the most underrated play in modern history. I found it positively Shakespearean. It's at least as good as Who's Afraid of Virginia Woolf?
Love The Lion in Winter- it is very underrated
Marat/Sade, good stuff.
Luther was full of verbal fireworks.
Who's Afraid of Virginia Woolf is a must see.
Naturally 
A Man For All Seasons, good but could have been better.
Becket, I actually enjoy this one more than the famous A Man For All Seasons.
The Miracle Worker, who would have thought the subject could be so entertaining?
Sunrise at Campobello, powerful weak.
Long Day's Journey into Night, the greatest play of America's greatest playwright.
And the most upsetting
The Crucible, Death of a Salesman, All My Sons- Arthur Miller's golden age.
Cat on a Hot Tin Roof, A Streetcar Named Desire- Tennessee Williams shows us how it's done.
Harvey, funny and heartwarming.
Read everything Samuel Beckett wrote, he is the supreme 20th century playwright.
As for the French: It's been forever since I read Jean Genet's The Maids, but I remember liking it.
Very kinky Ionesco's The Rhinoceros was painfully unfunny. Jean Anouilh looks interesting, but I haven't seen his stuff.
Did a good version of Antigone
What I've seen of Dario Fo's plays is very promising, but not as good as Pirandello's Six Characters in Search of An Author.
Gotta love metatheatre