Boris bikes? Should in principle be quickest for many journeys.
Once upon a time I tried to figure out how to take one out. Couldn't see any lights on them: do I need to bring my own? And it wasn't entirely clear how to pay. Nor what happens if I get one out and it turns out to have a problem.
 OK, not very hard. It was dark and cold at the time, and in reality I'd left just enough time to walk.