Not what will *you* remember him for necessarily, but when the minutiae of his mayoralty is forgotten in years to come what will he be remembered for?
It won't be something like 'the housing crisis' or crime rates or anything like that because it's too generic and relies on too many factors beyond the remit of the Mayor's office. I reckon that in the public consciousness (whatever that is) it will be:
Introducing a 24 hour tube network
The cable car (this will be remembered negatively)
Routemasters/Boris bus (maybe)
And the time he got stuck on a zip wire.