Visitor," "In the Pale Moonlight," "Far Beyond the Stars," "Trials and Tribblations," "Duet," "Move Along Home." OK, maybe not that last one...but man, DS9 just had the edge on exceptional episodes. Every series had some exceptional ones, don't get me wrong, but DS9 just really knocked it out of the park more than any other series. TNG had "The Inner Light," "Yesterday's Enterprise," "The Offspring," and several other good ones...Voyager had "Living Witness" (and not many others I can think of I'd consider exceptional).