The longest TfL contract route in London is the X26 from West Croydon to Heathrow Central, which is 21 miles long.
However if you include other routes running into London from outside, the longest is the 402 at a staggering 29 miles from Bromley North to Tunbridge Wells via Sevenoaks.
Despite recently having it's frequency slightly reduced by the advent of bendy buses, route 38 remains the most frequent route in London with an average frequency of every 3-4 minutes during the peakj periods.