Star Trek: Voyager
This was one of several episodes Voyager got to much quicker than fans might have expected (B'Elanna Torres being split into her human and Klingon components in "Faces" was another). It's a holodeck episode, which was one of the more notorious story types from The Next Generation. But in Voyager there's a unique reason to have a holodeck episode: The Doctor.
No, I didn't randomly switch topics again. The Doctor, not as in Doctor Who but rather the Emergency Medical Hologram, who in the early seasons was confined exclusively to sickbay because that was the only area of the ship equipped to project him. Except, y'know, the holodeck. So this was the episode where that finally happened.
"Heroes and Demons" might otherwise be known as the Beowulf episode, because that's the holodeck program that ends up being corrupted and needs to be played out. And the only person aboard the ship qualified to complete it is also the one who seems least qualified: not only an individual who technically did not exist until he was activated in the wake of the ship's catastrophic entrance into the Delta Quadrant, but an apparent balding man who when he isn't being acerbic is actually quite nebbish.
In other words, this is the episode that establishes what The Doctor is truly made of, and I'm not talking about his programming. (Much like the Moriarty hologram from Next Generation, "Doc" seems to have gained sentience simply from the unique circumstances in which he first appears. One minute he's uttering, "Please state the nature of the medical emergency," for the first time and the next he's being told that everything he knows about his programming is in fact being thrown out.)
Yes, this is the first time they tease The Doctor getting an actual name and no, it never happens in seven seasons (in the series finale he does settle on one, in an alternate future timeline).
Most of The Doctor's early development comes from his unique relationship with Kes, another apparent innocent with a few tricks up their sleeve. Of all the original characters from the series, they most embody the fresh start envisioned in the pilot (Tom Paris and certainly Torres are their closest rivals, until Seven comes along and kind of blows everyone out of the water, which in hindsight was probably appropriate, considering she was a bombshell) and undergo the fiercest trials during their time. Just when Kes seems like she's gotten her peace, she comes back a couple seasons later in the controversial "Fury," an episode that actually encapsulates the true legacy of Voyager quite well (you understand, not in a negative way; I love this series).
Two additional standout Doctor episodes would be "Latent Image" and "Author, Author," both of which completely deconstruct Next Generation's more famous "The Measure of a Man."