I think this poll sets up a hard choice because it all depends on how one interprets the word quality.
If one goes with a meaning of quality as the general standard or grade of something, well then the pre-Barn eps could win hands down because there was a much stronger emphasis on character, continuity and general story telling at that time than there was on DS in later years when certainly, as ProfStokes quite rightly points out, things like continuity went out the window and character was sometimes sacrificed in favor of plot.
However, if one interprets quality as the show's essence, then the post-Barn period could win out because the predominant supernatural themes brought an excitment to the show that was certainly different and probably more heightened than it had been previously to turning supernatural. Though to complicate matters, the supernatural elements were introduced on the show prior to Barnabas' arrival and they were well incorporated into the show during that time, which wasn't always the case post-Barn.
I believe that the epitome of the series was the 1967 phoenix storyline. It successfully blended realistic reactions and day-to-day issues with a compelling supernatural mystery. The early Barnabas episodes are like this too. I think if the writers could have maintained this level of writing, with attention to character and detail, DS could have been even better than what it actually was.
I agree that if DS had only maintained that level of writing, it could have been even better. But sadly, once Art Wallace left the scene, none of the other writers could stand up to DC in the way that Wallace was apparently able to.