The first thing to note is that London is served by five airports: Heathrow, Gatwick, Stansted, Luton, and City Airport, listed in descending order by passenger traffic. Heathrow sees more passengers per year than the other four airports combined.
City Airport is the closest to downtown and Heathrow is not much farther out, but the other three are a considerable distance from the city center.
Here's how I would rank the five airports, from worst to first:
Pros: Hard to think of any, really.
Cons: Super inconvenient.
There's no worse feeling than returning home from a weekend trip only to remember that you're on a flight going into Luton. All of the other airports are well-connected to London's transit network. Luton pretends like it is, but it isn't. When you exit the airport, you have to stand around waiting for a shuttle that takes at least fifteen minutes to get to the nearest train station. On Sunday nights, trains aren't very frequent, so you're often in a rush to catch one only to be thwarted by the shuttle. Also, it's quite a distance outside the city.
Pros: More convenient than Luton, quick to get from front door to gate
Cons: Long distance from the city, only accessible from one central train station
The main knock on Stansted is that you have to access it via London Liverpool Street, and the train ride out there takes close to an hour. The airport itself functions decently well.
Pros: Many more destinations than Stansted and Luton, fairly well-connected to downtown, good restaurant options
Cons: Still far from downtown, poor layout, feels a bit dated
Gatwick is the world's busiest single-runway airport. It certainly feels crowded in the main passenger areas, but the gates are very spread out (and a lengthy hike). Also given the congestion and just one runway, we have been delayed getting into Gatwick more often than at the other airports.
That becomes a problem when you have a flight that's supposed to land at 10:00pm but instead lands at 11:45pm. The trains to the city stop running and you're forced to take an hour-plus taxi ride. The last time this happened to us, the taxi fare was £111.
2. London City
Pros: Convenient and quick
Cons: Very limited destination list, only for small planes
The top two airports are leaps and bounds ahead of the bottom three. London City is excellent, first because of its location on the tube network, and second for its size; you can get there quickly, breeze through security, and arrive at your gate in hardly any time at all. It's perfect for the times when you need to travel to one of the European cities it serves, however that list isn't very long. That's because the runway is too short to accommodate heavy fuel loads, so it's dominated by small planes doing short journeys.
Pros: Far-reaching destinations, feels incredibly modern, always immaculate, great transport, many dining / shopping options
Cons: Occasional long lines at immigration, expensive to get there by express train
Here's why Heathrow is the best, summarized in one image:
You can get to any of these places on a direct flight from Heathrow - non-stop to just about anywhere in the world except Australia. Watching the departures board is mesmerizing.
A couple other things I like about Heathrow: There are grand pianos scattered around the airport with instructions to "play me", it has perfected the centralized waiting area concept, the multi-level layout is very well designed, and the bathrooms are as spacious and clean as you could ever expect in a public place.
These five airports are constantly discussed in the London newspapers. Ever since we moved here (and I'm sure for a long time before), there's been an ongoing conversation amongst politicians, lobbyists, city planners, environmentalists, and others about the current state of London's airport capacity.
The prevailing idea is that based on population growth forecasts, by 2030 London will need at least one additional runway to handle the increase in traffic. Amazingly, Heathrow somehow handles its current passenger volume (third in the world) with only two runways.
Mayor Boris Johnson's proposal to address the issue - a £50 Billion new island airport in the middle of the Thames estuary - was recently rejected by the UK Airports Commission.
Remaining are three competing proposals, two which involve expansion at Heathrow and one which calls for an extra runway at Gatwick. I am pulling for Heathrow expansion.
The core issue is that two runways simply aren't enough for the number of flights that Heathrow handles - for comparison, Chicago O'Hare has eight runways, Atlanta has five. An extra runway at Gatwick might be useful, but very few people connect through Gatwick like they do at Heathrow, so the benefit wouldn't be realized by nearly as many international travelers.
No matter which option is selected, the expansion isn't expected to be completed for another ten years, so it will be a while before London's airport configuration changes in any significant way.