I'm going to say the PATH system. Love it or hate it, I'm going to venture its the best indoor retail/commercial network out there given the size, quality of retailers, consistency of the quality of the network, and support from the city. Thoughts? Montreal's underground city doesn't come close. (I get it that many people live here their whole lives and almost never set foot it in it.)
I would think the most common answer though is multiculturalism/diversity. And when you ask anyone what that means they'll probably say restaurants and street festivals. Which I feel is superficial. And many cities are diverse. Pretty much every big city in North America nowadays is diverse. Toronto is just more diverse than most, if not all.