I love London's Underground system. It's fast, it's clean, it's convenient, and it's a snap to navigate. Many of the stations have pleasingly amusing names (how can you not enjoy getting on a train whose final stop is "Cockfosters"?); the Central line alone stops in Barkingside, Fairlop, and Chigwell. The conductors tell you to mind the gap and there are Cadbury's vending machines in most stations. There's something quaintly anachronistic about the stations. The official Tube website has a quirky little online poll on Tube etiquette. And, like most public transport, it offers excellent peoplewatching opportunities.
Sure, it has its quirks. The routes from train to the way out and between lines at a single station are often labyrinthine. The lift was broken at Earl's Court on the day I had to drag myself and my suitcase back Heathrow (but, thankfully, not the day I arrived, when I would have had to drag my suitcase UP those stairs). The 7-day transport card I'd ordered ahead of time and had shipped to me in California was demagnetized, and also some outmoded form, and nobody I asked at 5 different stations could fix it or replace it, so I spent a week having to be specially let through gates by (mostly cheerful) transport employees. My husband's card, ordered at the same time, worked fine.
Still, if it's not my very favorite public transport system, it's certainly in the top 3.
And this site has a bunch of creative variations on the Tube map, including my favorite, the Musical Map.