Several Star Trek entries, both original and Next Gen:
"Doomsday Machine" -- probably the TOS's finest space action story, with William Windom as a 23rd-century Capt. Ahab determined to destroy his white whale, the miles-long planet-wrecker of the title. If there's one episode that should have the modern effects -- good as the originals were -- it's this one. The scale is astounding. At one point Kirk is watching the planet-killer on the viewscreen of Windom's ship, and we can see, like a fly buzzing around a bull, the Enterprise . . . and Kirk's ship, we know, is nearly a thousand feet long! That planet-wrecker is BIG!
"A Matter of Honor," in which Riker takes part in an officer-exchange program: He volunteers to serve as first officer on a Klingon ship! Definitely not the kind of story the original would have done, at least not with the Klingons as enemies and rivals, as they were painted in TOS.
"The Measure of a Man," in which Picard, on behalf of Data, demands a hearing to decide whether Data is Star Fleet property, a slave in essence, or has the right to refuse a cybernetics professor's plan to disassemble him to find out how he works. A very Asimov-like, "I, Robot" kind of tale (the original, not the recent movie). It features no less than two scenes where Jonathan Frakes (Riker) proves he is not just a big stiff with a neatly trimmed beard, but a real actor who can make you realize what his character is thinking without pronouncing a single word. One of the best of ST: TNG.