The Darmok episode was a neat take on the differing grammar structures of an alien culture, and the difficulties of trying to communicate with a race that speaks in citation. The episode Q, aside from introducing this omnipotent Q guy of unspecified origin, introduced an interesting (and incredibly pop-culture prolific) antagonist collection of species known as the Borg, that seek the universal assimilation of highly intelligent sapient life without know reason. I personally love alien parasites, so the Borg were a fun topic of discussion.
Again, I don't have a lot to say this time around, other than how much I enjoy Star Trek in execution, even if some episode are a tad too much for me. Now to go subvert a trope.