The first subway system I rode was the London Tube system back in 1990. A very old system, my great grandfather worked on the tunnels before moving to the United States before World War I. I fell in love with being able to get near to everyplace I wanted to be in the city, quickly and inexpensively.
A few months later we went to Paris, another amazing system. My traveling companion was not as enthusiastic as I was, it was nearly the last gasp of that relationship.
Then came the Washington DC system, the sunway one of the reasons DC was on my short list of places we wanted to live. We live a 10 minute walk to from nearest station.
Chicago was next, the CTA Blue line is the best way to get from O'Hare airport into downtown Chicago, and the system will take you around the city with amazing efficiency.
San Francisco, the SF system is a little more complex and I wish it covered more of the city, but it is amazing. Unless it is the middle of the night, it is fastest way from the airport into the city. I landed in San Francisco near midnight one night, took a taxi into the city - 100 miles an hour in a Prius, a wild ride.
Philadelphia has a good system, actually two or three that don't play well with one another. A hard system to figure out, but a good system.
Portland has a nice line from the airport into the city, as does Seattle.
Los Angeles has a limited system, nice newer, modern. I rode it from West Hollywood to Union Station to go to the home of the French Dip Sandwich.
Toronto has a nice system, I have used it a couple of times.
The photo above is the entry to the Hudson Yard Station on the 7 Line in New York City, it reminded me of the entrances to the London Tube.