At the moment for me it's Muswell fucking Hill, I have attempted to scale this fucker about ten times and it fucks me up by the last third, and this is with gears. Or maybe I'm just a pussy....
I just came back from an Alps tour. Muswell Hill is like an exaggerated bump in the pavement now. And to be fair it's really not that bad, like Zack said try Highgate Hill. Plus there's a lush pub at the top for the victory Fruhli.