The 464 is certainly scenic.
There are several semi-rural routes in the Havering, Essex, area, but unless you wanted to be exposed to the (not entirely questionable) delights of Brentwood, not sure I would recommend any of them - they are mostly main-road focussed.
Slightly back into town, the longer version of the East London Transit, the EL2, might be worth considering: the final stretch out to Dagenham Dock, across marshland and former industrial sites, is pretty interesting, and surprisingly scenic in its own way. In fact, for sites, I'd recommend going around on the routes around the various docks areas, in Newham, Tower Hamlets and Southwark: some of these neighbourhoods really are like nowhere else in London.
I'd also recommend the H2, which I think is Solo-operated, for a tour around the delights of Hampstead Garden Suburb. In fact, if anything, the H3 (which I think only runs once an hour, and finishes unusually early for a London route) might be more interesting, going not just to the Garden Suburb but also across the Heath Extension, and then along the "millionaires' row" of The Bishops Avenue.