I think Spectre of the Gun looked fantastic for something that was obvious low budget. It is an example of what you can do with a low budget in a creative way. It is the same with Empath for me. Those two episodes are definitely my favorites in terms of look & atmosphere.
Spectre of the Gun is also my favorite of the alternative world episodes. Well I guess it wasn't pure alternative world because it was all in the mind, but that is why I liked it. Best explanation for doing a genre piece in Star Trek. And not to mention I always felt Trek already had a bit of a Western feel to it.
Another nice thing about the episode is it included Scotty and Chekov instead of some no name character (plus Chekov not Kirk got the girl).
One of my favorite third-season episodes, had a great Twilight Zone feel to it.