Favorites in the current era:
- Paris. Covers the whole city beautifully. You're always near a Metro station.
- Hong Kong. Love Oyster, also super-easy for the visitor.
- Tokyo. A great system, kind of a spectacle of humanity, and you feel like you've leveled up your public transit skill if you successfully figure out how to use it.

Oddball systems I like just because of the cool streetcars: Vienna, Lisbon

Sentimental attachment: Chicago, London. The first two big cities where I spent a lot of time on public transit starting as a teenager through my early 20s. Sadly neither score high marks globally in terms of cleanliness.

Systems I admired for their precision: Moscow and Washington. I loved the Moscow stations and the fact that they ran on-time to the second. (Not sure if this is still true - my visits there were in the 90s.) Washington Metro was the pride of US systems for a long time but I understand it's fallen into disrepair in recent years. Since the advent of Uber/Lyft, I haven't used this system much on my trips to DC.

Best airport rides. I give credit to places where the train beats traffic, or where the train is the best option for a visitor. HKG, ORD, BOS (efficient bus in a dedicated lane that connects directly into the T and Amtrak in Boston), SEA at rush hour.

Most mind-blowing station: definitely Shinjuku. But I also like it when the subway station shares space with an iconic train station, as happens often in Europe as well as in NYC at Grand Central Terminal.

Most frustrating that the such prominent airports don't have direct access to their city: New York. Taxis/Lyft/Uber still beat transit to Manhattan, which is kind of sad given how bad traffic can be. I've used transit from JFK and EWR on occasion, taking connecting trains, but it's really sad that there's no easy way in from LGA.