I suppose it's not
that unusual, but my nomination would be the daily Cardiff Central - Manchester and return. It does a double-back between Filton Junction and Bristol Temple Meads in both directions, only calling at Filton Abbey Wood in one direction (admittedly the correct direction for the morning/evening peak flows).
It's unusual in that anybody wanting to do that journey end to end would be much quicker changing at Bristol Parkway to/from one of the FGW trains towards Cardiff/Swansea.
With the double-back in mind, I know holders of tickets between Cardiff and points north of Cheltenham are able to use this service to/from Cardiff (direct train rule) but can anyone speculate on validity for holders of tickets to other locations?
Edit - these are the runs made by this one:
5M25 Barton Hill - Cardiff (ECS - runs through the station, reversing to the west, returning to the eastbound platforms - this is usual for the Turbostar services too)
1M25 Cardiff - Manchester
1V65 Manchester - Cardiff
1C00 Cardiff - Bristol TM