It could be the original 32 which was a circular route around the outskirts of Edinburgh until it was split into two. It was also renumbered as 32 for the inner circle and 52 for the outer circle. When split it came the 18 which is now the 200 (southern part) and the 21 (northern part).
For historians, the
timetableworld.com site has Eastern Scottish timetables for 1969 and 1978 which have lists of Edinburgh ECT/LRT routes with first and last times but not frequencies. The outer circle came in after 1969 (there was no Wester Hailes then) so for the fifties and sixties the east-west 26 and 44 Eastfield-Clerwood/Juniper Green may have been longest linear route.
The 32 started in October 1971. It wasn't a circle initially, running between the Foot of Leith Walk and Wester Hailes via Newhaven, Granton, Muirhouse, Drylaw, Blackhall, Drum Brae and Broomhouse. Subsequent changes saw it extended, from Wester Hailes to Hyvots Bank and from Foot of Leith Walk to Hay Drive, before becoming a full circle in April 1975.
The change to 32/52 for the inner and outer circles took place in March 1981.
In January 2002 the 52 was renumbered to 32A with the split into two linear routes, 18 and 32, happening 18 months later.