Favorite Star Trek: The Original Series episode?

I felt like watching TOS, and put on one of my fave episodes, The Immunity Syndrome.

It's a very good sci-fi episode, where the characters are faced with something so perplexing they don't know how to handle it, and have to learn and understand their way through the first half of the show.

Once they figure it out, it goes from Sci-Fi to character drama as they need to figure out who is best to deal with the problem and the reactions to possibly losing that person. And finally a last ditch fix that could legitimately end with the deaths of everyone while attempting to save the universe.

The effects are also pretty good for 1968.

It's a really great episode among great episodes, and for me, the one that stands out when I think of TOS.