TOS: "Mark of Gideon" - Just because the concept is so absurd. An overcrowded planet, yet they can build a full-size recreation of the Enterprise???
TNG: "Masks" and that awful one with Alexander and Troi's mom in the mudbath. The whole first season is also pretty hard to watch now. The whole concept of Starfleet being perfect and everybody getting along made for ham-fisted dialogue: "I'm with Starfleet. We don't lie." Ughhhh!!
DS9: Couldn't think of one until I saw the post about "Profit and Lace". Agree 100%. "The Storyteller" I think is the title of another one that bores me. 1st season episode where O'Brien has to tell a story to stop some cloud from attacking people on a planet.