No series before or since has gone as dark as DS9, what with the long drawn out wars (plural) where they spent episodes staring at the names of the dead, a final war they basically lost if not for doing many illegal means to win it, a war where Nog lost a leg and has to learn to deal, Jake watches men killed in front of him, Bashir's parents go to prison for being criminal, Dax is murdered for no reason while praying (!!), Section 31, every episode with Kira (she's a terrorist whose mother slept with the leader of the occupying forces!) or any other episode featuring O'Brian in any way, shape or form (name a good moment he ever had, I can't think of any), our beloved spy Garak torturing Odo for 2 episodes straight, "In the Pale Moonlight", War war war war war.
The goofy light comedy episodes where Quark dresses as a woman did nothing to change the overall tone of the series, it was as dark as Trek gets, even to this day.