I don't know that I can choose a best, but by far and above the WORST: Death Takes a Holiday.
Best: The Normal Heart, Peter and the Starcatcher
Worst: The Addams Family
Best:
Les Miserables (West End)
Catch Me If You Can
GHOST
Billy Elliot
War Horse
Worst:
Addams Family
Priscilla Queen of the Desert
Lysistrata Jones
I also thoroughly enjoyed the Evita revival that is currently on Broadway.
It's been two years and I stand by my original answers of Ragtime and Cats, respectively, but in the Best Category I have to give a nod to The Normal Heart.
Non-NYC:
Best: Uncle Vanya (Sydney Theatre Company when it was playing at the Kennedy Center)
[Sidenote: you must see this when it goes to Lincoln Center this summer.]
Worst: Fuddy Meers (regional theatre just outside DC)
New York:
Best: tie between Next to Normal and Jerusalem
Worst: tie between Mary Poppins and The Pirate Queen
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