Maybe if .London took off like .ca I would probably go against the grain.. Without researching and all were the same price and I had to choose, probably london.taxi because the other two are backwards. The com may sell but it's a rare occurrence. Or just make a decent offer on the premium name londontaxi.com
A 'London Taxi' is used colloquially to refer to 'Hackney Carriages' (these are the make of taxi that is often seen in films and on TV and are normally black in colour). It takes years of training and to pass many exams for a driver to finally get their 'Hackney Carriage Licence' (there are actually two types of licence, 'All London' and 'Suburban').
In London therefore when someone is referring to a 'taxi' this is what they mean, a 'London Taxi'. Indeed now that Hackney Carriages are used in cities in the UK outside London they are still referred often to as 'Hackney Carriage' or 'London Taxi'.
Just as an aside, mini-cabs are not referred to as 'taxis' in London but rather as 'cabs' - a distinction that is actually legally based.
Hope this explains why 'London.taxi' is by far the better domain.
(I have a few friends who are or have been 'Taxi Drivers' in London and spent many an evening helping them with their 'Knowledge' (learning routes and major buildings, etc. ready for the tests they undergo.) )