Mmm, sounds like a great study.
I find the ones in NYC pretty bland wrt to others aroudn the world, but the one we go to is Jin Fong in Chinatown. I understand the good ones are to be found in Flushing, but have not found them to be any better. Ping's is also good, though I was told by a colleague that she will not go there anymore as they found a cockroach in the dumpling!
I find the Toronto ones rival the ones in Hong Kong. I have listed two cart service and two menu service ones
Cart - Spring Villa (in agreement with Rejuv) and Bright Pearl (downtown Chinatown)
I think Lee Wah Heen is one of the best dim sum places, but also put Dynasty as a worthy rival.
I have been to a few in HK and Beijing that are worthy of mention but will have to dig up the names.