Okay, actually trying to compose a faithful list--including ones I haven't seen but considering what the consensus on them seem to be.
I'm gonna have to focus on musicals because I don't know nearly enough dramas, but Pillowman and Doubt have to be on any lists including plays.
No particular order:
Sweeney Todd
Light in the Piazza
Dirty Rotten Scoundrels
Hairspray
Urinetown
The Full Monty
Avenue Q
Too many to decide between for the last 3, especially considering some i haven't seen, but I'd consider: Spelling Bee, The Producers, Wicked, Caroline or Change, 42nd Street revival, Assassins revival and Pajama Game revival
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli