I'm a huge fan of Addison Lee, especially since they have implemented their online booking service. Now it couldn't be easier to order a taxi. Living in East Dulwich, black cabs are just much harder to come by, so Addison Lee is a saving grace.
The online booking service is also fantastic - you can save off your favourite locations to make repeat bookings super easy.
They also a lot of cabs waiting around the mainline train stations, so rather than pay the premium of a black cab, I prefer to get an Addy Lee to meet me.
Drivers are courteous and the cars are clean. In my experience, they have been on time 99.5% of the time.