For me, with regards my Ripley anyway, it is John Ostler.
Great chap to talk to and when I dropped mine off to him (drove up from Portsmouth) he took one look and said 'I built that one.' . When I collected it he let me put a few shots over the chrono with the pellets I use to confirm the fps I had asked for then showed me all the bits he had replaced.
Admittedly had a firing valve snap about 4 weeks after that which was replaced FOC. Thats a better service than I have had from some garages.
Since that time it has only needed a little tweak downwards on the velocity a year after that as was getting nearer to 780 than my preferred 770fps. Other wise has performed faultlessly, unlike like its owner.
Will be taking my older Ripley to him for a complete service.
Can not comment on any others as not used anyone else.