I love cities with public transportation, and I particularly love cities with subways since that means I won't have to drive to get around. Montreal is the city with the cleanest subway I've seen of late although Toronto probably beats it hands down (I just haven't been to Toronto in the last 8 years). Montreal's subway cars are clean and well-kept, they run on time, there is never a long wait, and I don't feel unsafe traveling the subway after dark.
The service to the metro area is well-planned and I can get from one end of the city to the other in under an hour (and more likely under 30 minutes) including the walk to the destination. In a city of 4 million people that is as spread out as Montreal, this is key. The best thing I like about it? I see locals using it ALL THE TIME, so it's not just for tourists.
The only issues I have with Montreal's Metro? There are very few stops outside of the island of Montreal, meaning that there are places I can't see and things I can't do because the Metro doesn't run there. But I'll suck it up and deal with my disappointment, because Montreal's Metro is, hands down, the most well planned and executed subway I've traveled in a long time.