From where I live in north London all of the 6 London airports are roughly equal in being pretty easy for me to get to.
I travel for work and for leisure but don’t work in aviation. As a punter I know the differences between all 6 and I pay my money and make my choices. As I say, from a door to door perspective I am spoiled for choice.
Aside from London City which is a sinch to get to, Stansted by road or train ought to be the most accessible in terms of distance and time and therefore my preferred choice– but for leisure travel at least, Gatwick wins for me.
Why?
Train wise as
jdcg has posted – it's expensive. Also, if something goes wrong on the lines in and out of Liverpool Street the service seems to shut down and you are stuffed. Stansted Express trains always appear rammed full.
Driving – Gatwick has a better choice of parking options, and the transfers to/from the terminal are less painful.
As a passenger I also think that Gatwick has improved massively because of the investment from the new owners – so scope for future improvement at Stansted when the sale goes through, but not guaranteed. Luton has never been owned by BAA, but various changes of owner/operator have never IMO positively impacted on the experience of using Luton as a passenger. Its not a foregone conclusion that removing BAA’s involvement at Stansted will see everything sorted for evermore.
As for Cross Rail. In just over 5 years time Cross Rail is going to be a game changer across London and the south east of England, and will reduce the time it takes to get to all sorts of places (and interchanges) for a very large number of people. Example - London City to Heathrow in not much more than an hour.
Cross Rail might mean that its easy for more people to get to Stansted – it will also mean that it will be easier for passengers who only currently use Stansted to fly from somewhere else.