The Happiness Patrol
This is the only Seventh Doctor-era story that seriously disappointed me. (I have seen all but Delta and the Bannermen.)
It claims to be an satirical and intellectual tale of a population being denied the right to feel sad, and to some extent it is; but, it comes off as fluff--not very intellectual, witty or funny, but just silly. Granted, the Kandy Man (an evil executioner who is literally made of candy) is /vaguely/ scary, esp. when s/he snarls, "Welcome to the Kandy Kitchen, gentlemen...I like my volunteers [to die] with...THMILES on their faces!", but most other characters are dismissably dull. And some of the things that happen during the 'revolution' against happiness make the viewer groan and say 'Riiiight'--such as the Doctor dousing the Kandy Man with lemonade, which apparently reacts with his candy-flesh so as to make his feet stick to the floor.
Another reason to dislike The Happiness Patrol is that unlike most Doctor Who stories, this is overtly and offensively politicized. What the political messages are is not relevant, but I will say that the reason I dumped Star Trek is that I had grown tired of Trek's arrogant and incessant attempts to indoctrinate its audience. Doctor Who has consistently avoided shoving lessons down the viewer's throat, but The Happiness Patrol does exactly that. Really, the only other such exception in Doctor Who is The Green Death, a laughable serial about capitalists who pollute the environment because they are bad, bad, bad.