The Greenford branch line seems less than ideal, especially now its a bit isolated from other GWR local services.
How many shuttle routes are there anyway? Some definitions suggest no intermediate stops, but assuming that we allow a small number of intermediate stops, and also require few/no through services, likely to be only part of most journeys rather than the whole thing, and a short distance, I can think of:
Bromley North-Grove Park
Greenford
Stourbridge
Aylesbury-Princes Risborough
Lymington
Marks Tey-Sudbury
Butetown/Cardiff Bay (but I believe this is supposed to be served by Valleys through services in the future)
GWR Henley, Marlow and Windsor services
I guess the Looe Valley and St Ives lines could count too. I feel that Par-Newquay is too long, although most travellers will use the full route and it will only be part of their journey.
In all cases I'd say they're satisfactory in one respect - they exist! So many similar services no longer do and don't always have good bus services.