If you are looking to get a sense of just which place is the hottest in summer, the answers above speak to those clearly. For me it's is more about the most moderate climate, the answer is very different. A better questions would be, what is the warmest place in winter? or perhaps which has the most moderate climate.
E.g. several places in Canada go as high as (and even above) 35 degrees C in summer, however a large number of those places go well below minus 25 down even to minus 35 in winter.
Canada is is place that have weather extremes. Just because it is warm in summer does not mean it will not get really cold in winter.
I have found that when you look at average temperatures over a period of a year (winter and summer and there are stats available online) this will give a better sense.
Having lived in colder and warmer climates in Canada, here are my thoughts :
Vancouver is one of the most moderate around. Hardly cold at all in winter, and not too hot in summer. Lots of rain though so you will likely have to resort to taking Vitamin D supplements to stay same.
Halifax colder in winter (lowest around minus 20 average -5) and lovely in summer, hardly ever over 30 degrees but with humidity.
Central canada, most of the places get fairly hot in summer but also very cold in winter.
Coastal cities have some moderation in climate due to ocean temperatures that tend to stabilize the coastal temperatures.
Downside of a moderate location. If you like snow, and want to get into winter sports (and there are lots), you will have to travel to get to snow.
Tough choices. Good luck