This is a real tough one. For my money series 8 saw Summer Wine in its absolute pomp, the best series of all. Each episode was a cracker, well written and beautifully executed.
In fact I'd say that all of my fave ever episodes are in this series. It's a close one between Keeping Britain Tidy and The Mysterious Feet and I'd just go with The Feet I think, although I do love Who's Looking after the Cafe aswell..."a bit tucked away in a Eunuchs Truss"...