Where you are staying in London can matter as much as the airfare. The same airport may be convenient from one side of the city and awkward from another.
West London
Heathrow is usually the natural choice for west London and the Thames Valley because it can be reached without crossing the city.
South London
Gatwick is often strongest from south London because of its rail links through Victoria, London Bridge, Blackfriars and the Thameslink corridor.
North and northwest London
Luton often works well from the St Pancras/Thameslink side of London and the M1 corridor. Stansted can still win when the flight choice or fare is much better.
Northeast London and Essex
Stansted has the strongest broad route network in this direction. Southend can be convenient when it happens to offer the required route.
Canary Wharf and the City
London City is exceptionally convenient for the Docklands and financial districts. Its smaller route network means that convenience has to be weighed against flight choice and price.
Central London
There is no universal winner. Heathrow, Gatwick, Stansted and Luton all have useful public-transport links into different parts of central London, so compare the exact hotel or meeting address rather than the distance from the map centre.