I was going to say Montreal is clearly the obvious choice over Toronto for a visiting American who has to pick only one, but it looks like I didn't need to weigh in.
At the opposite end of the spectrum, I had a friend from Edinburgh visiting and my first reaction was to take her to Tourist Spot #1 in Eastern Canada, the stone-built walled city of Old Quebec, then it immediately struck me that it was the least exotic location I could have chosen. For a Houstonian though, it would have been the other way around. My Scottish friend would have found the six lanes in each direction of the 401 freeway to be a truly exotic sight... meanwhile I'm sure the OP would not.