Traditionally, "Far Beyond the Stars" and "In The Pale Moonlight" are considered among the highest quality eps. "The Visitor" is usually also considered one of the best. But it really depends on which ones you like. Everyone has their prefrences. Also "Trials and Tribble-ations" is a classic one, too.
My favourite was "Take Me Out To The Holosuite", because it featured a rendition of the national anthem of the United Federation of Planets.
The episode "Way of the Warrior" has some of the best battles from Deep Space Nine.