It would, without a moment's hesitation, be "Marco Polo". With "Fury From The Deep" and "The Web of Fear" close runners up.

I think it is fair to say that it is still "mainly" good - it is just the number of "really great" episodes declines, and you do start to get a few "quite stinky" stories.

In part, I think that's because the companions aren't as good post Ian-and-Barbara, so the actors don't sell the less-good stories so well.