Neroon, the problem is that "episodes that aren't often listed as "the best of the series"" is so subjective, that I wouldn't know where to draw the line. Do you mean episodes that never make it into people's top 10 lists? Their top 20 lists? Their top 40 lists?
Maybe you should first come up with a list of episodes that are disqualified because they're too popular, and then we can offer suggestions on the best of the rest? Or else, we can collectively first do the list of popular eps, so that we're then all on the same page as far as what counts as a hidden gem.