My favorite US cities in no particular order are Honolulu, San Diego, Austin, New Orleans, Nashville, Charleston and Memphis.
I probably wouldn't want to live in New Orleans (I'd die from partying too much) or Memphis (it's got a ton of character but it just isn't that nice) but the rest of them are good places to live too. A lot of people really like Denver these days which is where I live but I know too much about it to rank it high on the list.
Canada has 3 great cities that are comparable and probably better than its American counterparts (Toronto, Vancouver and Montreal are right up there with NYC, Boston and Seattle) but I prefer warmer weather.