Where London is genuinely world-leading
Payment is the clearest win. Tapping a contactless bank card or a phone at the gate works across the Underground, buses, trams, the DLR, the Overground and the Elizabeth line, with automatic daily and weekly capping so you cannot overpay. Visitors do not need to research fares, buy a card, or understand zones. Very few large cities have solved this as completely, and travellers arriving from places where you still queue for a paper ticket notice immediately.
Coverage is the second. The combined network reaches an unusually large proportion of the metropolitan area, and the bus system in particular is dense, frequent and fills the gaps the rail network leaves. Night services on core routes run every night, and the Night Tube adds weekend coverage on several lines.
Integration is the third. One payment method, one journey planner, one fare structure across modes owned and contracted by different operators. That sounds mundane and it is the thing most cities fail at.
Where it falls behind
Cost. London is expensive by European standards, and dramatically so compared with cities where transport is subsidised as a public good. A single journey costs several times what the equivalent would in Madrid, Prague or Warsaw, and the gap widens for longer trips.
Accessibility. This is the most serious weakness and the least visible to able-bodied visitors. A substantial share of Underground stations have no step-free access at all, because the deep tube was built long before anyone considered lifts. Newer lines like the Elizabeth line and the DLR are excellent. The Victorian core is not, and a wheelchair user or a traveller with a heavy case can find that a station on the map is functionally unavailable to them.
Heat and comfort. Most deep-level tube lines are not air conditioned and cannot easily be, because there is nowhere to put the heat. In a summer heatwave the Central and Northern lines are genuinely unpleasant. Cities with newer metros do not have this problem.
Luggage. The network was not designed for it. Narrow gatelines, stairs at interchange stations, escalators where standing with two cases blocks the left, and crowded carriages at peak. Tokyo and Singapore handle this better; Paris and New York handle it worse.
How it stacks up against specific cities
Against New York, London wins comfortably on payment, cleanliness, signage and off-peak frequency. New York wins on price and on running everything twenty-four hours.
Against Paris, the Metro is denser in the centre and cheaper, but London's integration across modes and its contactless system are better, and the Elizabeth line has no real Parisian equivalent for cross-city speed.
Against Tokyo or Singapore, London loses on punctuality, cleanliness, air conditioning and station accessibility. It wins on the simplicity of paying and on the legibility of the network to someone who has never been before.
Against most North American cities other than New York, there is no contest. London's weakest week is better than their best.
What this means for your airport journey specifically
Airport rail links are where the general ranking stops being useful, because the question is no longer about the network but about your particular door-to-door journey.
The rail links themselves are good. The Elizabeth line and Heathrow Express serve Heathrow well, the Gatwick Express and Thameslink serve Gatwick, and both are fast and immune to traffic. If you are one or two people travelling light and your destination is near the terminus, take the train. That is the honest answer and it is the cheaper one.
The calculation changes with every variable you add. Stansted and Luton have longer, less pleasant connections. A destination that requires two changes turns a clean rail journey into an exercise in hauling cases up stairs. A group of four splits a single fixed fare four ways while paying four separate rail fares. A late arrival runs into thinner services. And a step-free requirement can make an apparently simple route impossible.
The rule of thumb: rail wins on price and predictability for light travellers going somewhere well connected. A car wins on total cost and effort once you have three or more people, more than one bag each, an awkward final mile, an unsociable hour or a mobility need.
Using the network well as a visitor
Tap in and tap out with the same card or device every time, or you will be charged a maximum fare. Do not mix a card and the phone it is loaded onto, because the system treats them as two people. Stand on the right on escalators. Check whether your station is step-free before you commit to it with luggage. And remember that the tube map is a diagram, not a map: neighbouring stations are often five minutes apart on foot and a change of line away underground.