I like The Twin Dilemma too. The worst stories for me are Voyage of the Damned, End of Time, The Doctor's Daughter and Planet of the Dead. Absolute lowest common denominator stuff.
There's no classic serial I can think of that bugs me as much, or I find as obnoxiously anti-intellectual and stupid. Their worst crimes tend to be being boring (Hi, Galaxy 4!) or lazy (Bonjour, Timelash!). They never offend my moral sensibilities.