GMK Metropolis by Nephlock is one of the most visually arresting keycap sets I’ve seen, with bold colors on a dark blue base and symbology for transit enthusiasts. Unfortunately, Metropolis has been one of the most expensive keycap sets to get on the second-hand market, because it is so popular. Finally, in about a year, new copies of GMK Metropolis will be shipping again after people who purchase it before April 30th, 2021, receive their sets. GMK, the company that makes these keycaps, really needs the pandemic to be over.
If you find the colors to be a bit much, but enjoy the mass-transit symbology, there is a more subdued base kit with teal highlights called Midnight:
The regular Metropolis base kit is $120 USD, Midnight is slightly cheaper at $110 presumably because the kit is a little smaller and uses fewer colors. As with all of these Group Buys there’s a risk of delays if the
GMK Metropolis is available with more options, matching deskmats, and more, until the 30th of April at Novelkeys for North America, Desk Hero for Canada, proto[Typist] in the UK, Oblotzky Industries for the EU, Daily Clack in Australia, zFrontier in Asia, and monokei in South East Asia. You can read a bit more about the set in this thread from Nephlock on Geekhack.