Easily the fastest for me is by bike. I travel from SE London to Chancery Lane in London and as soon as I hit the Old Kent Road I'm passing motor traffic. The journey is just over 6 miles. By car it takes about 45 mins to an hour, by bus 1 hour or more, by bike 20-25 mins. Motorbike or moped may be quicker than the bike, however although they are a tad faster along the OKR (if the lights are against them though I catch 'em), they cannot sneak through the gaps in gridlocked traffic in central London as easily so get held up. Even when I run it I can get in faster than by bus, takes about 45-50 mins.
I think the only way I would beat the bike would be by helicopter