I've long been of the opinion that Star Trek: Deep Space Nine was the best series of the franchise, and one of those reasons is the character of Garak, played by Andrew Robinson. From his first appearance in the second episode of the series, Garak announced himself to be just "plain and simple Garak." In fact, that was far from the truth.
Deep Space Nine was a space station abandoned by the Cardassians, a race in a state of truce with the Federation, after they were finally forced off of the planet Bajor by the revolt of the populace. Basically, think of the Cardassian Military as the SS and Bajor as WWII-era Poland. Thus, the Cardassians weren't too popular on Bajor or Deep Space Nine after the evacuation - making Garak's continued residency an oddity that immediately made one wonder about his history.
Beyond that, Garak was often a good source of the sorts of "outsider" observations and often dark lines that some of the other characters couldn't get away with. There's also the fact that he so often bent the truth that eventually everyone stopped taking anything he says at face value. In this clip from Season 3's "Improbable Cause," Dr. Bashir uses "The Boy Who Cried Wolf" as an analogy for why no one trusts Garak when he seems to be in danger. Garak - in one of my all-time favorite lines of dialogue - offers a different interpretation. (The teleplay is credited to Rene Echevarria.)
Another excellent Garak scene is this exchange between him and Quark in Season 4's "The Way of the Warrior." Here, root beer is used as a surprisingly deft metaphor for the Federation. This episode was written by Ira Steven Behr & Robert Hewitt Wolfe