Mississauga
Ranked 1st among all North American suburbs in terms of high-rises, and 21st among all North American municipalities, ahead of Denver, Seattle, Minneapolis, etc.
That said, Mississauga isn't the king of suburban high-rises in North America. That honour goes to Toronto: the former municipalities of North York, Etobicoke, and Scarborough, and the Thornecliffe Park neighbourhood of East York, combined for a whopping 1000 or so suburban high-rise buildings, around 500 of which are located in the former municipality of North York alone.
Imagine if Toronto de-amalgamated, these suburbs would dominate the North American rankings. Toronto itself would fall to 3rd place below Chicago, but North York would be around 10th place, ahead of San Francisco, while Scarborough and Etobicoke would probably be at least 20th and 21st place. And of course Mississauga would be pushed down to 24th place.