I will nominate London, Paris, and Buenos Aires as leading contenders. New York is for me too familiar for me to judge objectively and so I exclude it.
Reasonable safety is a prerequisite, and then we have the following dimensions:
1. Chance of seeing a striking yet non-famous piece of architecture. All three cities are strong here.
2. The right mix of broad boulevards and narrower streets. Ditto.
3. The chance of spontaneously encountering good bookstores or excellent dark chocolate: London wins the former, Paris and Buenos Aires win the latter.
4. Cheap, convenient cabs, and places to sit and drink sparkling water: Buenos Aires is #1 on these.
5. Strangers are willing to talk to you: Tough to call, though NYC would win hands down if it were in the running.
6. Strategic and frequent use of historic plaques: London wins; yesterday I saw “George Canning lived here” and “Clive of India lived here,” among others.
B.A. loses points for imperfect safety and also capital confiscation, though it has by far the warmest weather of the trio. Overall I am inclined to pick London as first, perhaps because I prefer English to French for bookstores. Paris offers fewer surprises, even if it has a higher average level of beauty. Paris is also worse for spontaneous cheap dining in restaurants, though it has far better food stores for urban picnics. Berlin is perhaps the best city right now for living, but it is too spread out, and with too many broad boulevards, to be the best walking city. It is an excellent city to take a cab in.
Walking cities on the rise: Istanbul. I suspect it’s long been splendid, it’s now reaping the gains of being modern.
Underrated walking cities: Moscow, Mexico City, Toronto, parts of northern England, Los Angeles.
Overrated walking cities: Budapest, Krakow, Munich.
Best city to take the subway through: Tokyo.
If I had to pick a fourth in line: Barcelona.