Biggin Hill is certainly the closest to Canary Wharf, you can see the office buildings there from points on the field, let alone on approach. Closely tying is Stansted as there is a direct motorway between the two, it's further but the road journey is only about 35-40 minutes. About the same journey time from Biggin, on slower streets.
Luton and especially Farnborough are a long way away from Canary Wharf.
Advice differs for other Greater London destinations. Like many other large metropolitan areas, it depends on where you want to go.
Other option of course is to put the pax across to an operator who has the right types to get into London City ! In fact I wonder why they don't do this themselves unless you're beyond the range.