Number One Bus
As all London transport geeks know, the oldest bus route in London - and therefore almost certainly the oldest bus rout in the world - is the number 12, from Oxford Circus to Camberwell. It is the diredt descendent of the first motor bus route run by Tilling's back in 190-something, and that itself was a descendent of probably the first horse bus route. Though some argue for the 9, and there are a few running-dogs and revisionists and who hold out for the 24. Splitters! (And why does everybody on a southbound 24 in Goodge street in the evening look so sad or hassled?)
So why is the number 1 the number 1? It's not a particularly important route, nor a very long distance one. OK, the 12 was adopted into the London Transport numbering scheme a few years after it started, as it was being run by a private company who didn't yet use numbers - but what gave the Lewisham to Willesden route precendence over the numbers 2 to 9 - all of which still exist, and have at least some of their route in common with what they were doing a century ago.
Its just one of those mysteries.