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).