my nomination is 2P99 0325 Peterborough - Kings Cross via Hertford North using a 4 car 365
http://www.realtimetrains.co.uk/train/L01079/2015/01/13/advanced
despite what Realtime trains says about it being a 317 its definitely a 365 went on it the other week as i needed hertford north for track coverage and refuse to use a 313
Booked for a 365, and has been for as long as I can remember. MF the train goes on to form the first northbound Kings Lynn, which also has an unusual pattern, being fast FP to Stevenage, then all stations to Kings Lynn - the only northbound through service from Stevenage and Hitchin to Kings Lynn.
There's also a similar service in the evening, 0105 King's Cross to Royston (used to be Letchworth) via Hertford North, calling at some but not all of the stations on the Hertford line. Some time ago this was booked 1x317, but in the last few years has been booked 1x365.
Another slight oddity is that through the day the Kings Lynn trains are generally extensions of the fast Cambridge trains, in both directions. However from around 1800 the up Kings Lynn trains form the semi-fast services from Cambridge onwards. I presume this caters more conveniently for journeys such as Ely-Royston, however for others this is a pain, as it makes these trains busier than they would otherwise be south of Cambridge, they tend to be confined to 4 cars to avoid the need to join at Cambridge, and it provides a much slower journey for those travelling from Kings Lynn, Ely (etc) to London.
On the same line, there is an 0810 King's Cross to Peterborough which calls at Stevenage, St Neots, Huntington and Peterborough, and as 3x365. However this can be explained simply that this was originally an ECS move which recently was decided to be made a passenger service.
And on the subject of ECS moves, the 1422 King's Cross to Peterborough is formed from the 1230 Cambridge to King's Cross (arr 1332), however presumably because there is limited platform availability, the train runs empty to Hornsey reversing sidings and back to King's Cross. Really irritating, as it means the train doesn't arrive at King's Cross until a few minutes before departure time.
Finally, Saturday morning still sees one up service start from Huntingdon bay road. Can only assume this is for some kind of route knowledge or rusty rail prevention, as there doesn't seem to be any other obvious reason, especially now Peterborough has its additional platforms.