Difficult one to call. I've always had a soft spot for Paris (despite the smell and chaos) and Berlin (particularly the inadvertently preserved former "Ghost Stations" in the old East). Neither are perfect systems, but they have something particularly atmospheric about them.

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What are some of the most enormous subway stations that you know of? How do you deal with them.

I regularly have the pleasure of King's Cross St Pancras, a station with 6 lines going through (three are on the same tracks, but still). I have learnt back routes through the station, over the years, and that it's often a lot easier to walk on the surface than through the tunnels. I've also learnt when it's busy enough to just give up and go another way. When they artificially restrict flow into the station by half-closing the gates, you know it's not going to be a fun journey.

London stations have nothing on the massive Paris complexes like Montparnasse-Bienvenue and Chatelet-les-Halles, though. I just tend to avoid connections at these stations, unless I know they're on nearby lines, because the time lost in tunnels and stairwells isn't worth the more direct route.

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What is your favorite airport for subway access to the downtown area it serves?

Oh, Copenhagen, easily. Automated metro (bonus points for being able to sit at the very front), no premium fares, every 5 minutes, and right into Kongens Nytorv in under 15 minutes. Reliable enough that I often used to leave the city an hour before flight departure.

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Do you have any general tips to offer on the systems you know best?

The London Underground is an amazing system, but it's old, complex and hugely busy. Time it well, always check for disruptions, and take advice on easy interchange stations. Remember, though, people are sheep-like in crowds, and you can always get more space, particularly nearer the ends of the line, by moving down the platform - particularly to the extremities. There are fast and slow lines - generally, the subsurface lines are slower than the deep tube lines, and the newer ones (Victoria, Jubilee) are quickest of all (until the Brenda Line comes along). Don't assume that you're best off getting the Tube, though - there are lots of routes where you're better walking, or getting a mainline train.

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What's your protocol for identifying optimal station exits?

There are apps for this in London, which even go down to telling you the optimal carriage to be in for the exit! It depends how much of a rush I'm in, but in general, I like to familiarise myself a little the the area I'm visiting, so if I'm given an option like "High Street North (West Side)", it's easy to figure out. And if it's not the optimal one? Ach, well.