For me cities with great public transportation are key.
New York still has a lot of cars, but the subway system is quite extensive. Portland, OR has one of the best bus systems I have seen. I wish they had more light rails than they do. When I was last there they were just completing the second one.
Paris has a great subway system that is very clean, and their roads are very walk able.
Montreal and Toronto have great systems.
I think a cities climate is always important to consider. If you want to have bike paths, but you get a ton of snow in the winter, how do you deal with that? At my university up in Rochester NY we had tunnels under ground connecting the buildings. This helped a lot with the brutal winters.
IMO Toronto's system is not so great, especially for a city its size. A city that size (especially one with as much congestion!) should have a comprehensive subway system (see Barcelona for a great example), not a measly 2.5 lines.
Sure, downtown the transit is pretty good, but move outside of that, and things aren't all there.
I love streetcars, but since there are too many cars on the road, trying to get around downtown is a joke. That said I take the King which is known for being one of the worst I think. It's faster for me to walk a lot of the time.
I hope we can do better than NYC and Paris for public transportation. Paris's are smelly. NYC haven't been updated in forever.
I'm not trying to be negative, only pointing out there's better. On my short list I'd pick Tokyo, Kyoto, Osaka, Singapore, HK (maybe), even London's is pretty nice compared to Paris. It's clean (at least in my limited experience) and at least for convenience Berlin's is pretty amazing.
Paris has a great subway system that is very clean, and their roads are very walk able.
Montreal and Toronto have great systems.
I think a cities climate is always important to consider. If you want to have bike paths, but you get a ton of snow in the winter, how do you deal with that? At my university up in Rochester NY we had tunnels under ground connecting the buildings. This helped a lot with the brutal winters.