Hmmm... On Broadway: Seeing Yul Brynner play the King in a early-mid 80's revival of The King and I -- 3 times!
Seeing Hugh Jackman in The Boy From Oz
In London: being "introduced" to Michael Ball in Chitty Chitty Bang Bang... loved the show, and have become a fan of the original stage Caractacus as well

Also, seeing Vanessa Redgrave and Joely Richardson in Lady Windemere's Fan -- a night I will always remember, not only for their wonderful performances, but that was the night the Queen Mother died and the show started with a moment of silence.