@tsarstepan,
To answer the actual question: I've always been a partisan of
Star Trek. And although it's unfair to single out any one episode as my favorite, I'm going to pick
Specter of the Gun. The only reason I'm picking it is that I just saw a promo film for
Cowboys and Aliens, scheduled to come out on July 29th this year. Everybody there is boasting how the idea of merging SciFi and Western is utterly new and original---and yet,
Star Trek did it 42 years ago, and repeated it with every new incarnation from
TNG to
Enterprise. (Honorable mention for the
Firefly series (2002-2003), which was entirely based on this premise.)