DS9 and TNG for me, but DS9 just a bit ahead.
By VOY's season 6, I was so bored I wished the show would end. None of the principal characters had anything to do, and the desperate arrival of Lt. Barclay only confirms what all knew - the show was tired. TNG-free, but still trying to grab whatever TNG tried to do, with little to no success. Doing what no other Trek show had done before - force me to close the TV.
TNG, after a very so-and-so season 5, gets back on top with a really good sixth season. Episodes like "Times Arrow Pt. 2", "Relics", "Quality of Life", "Fistful of Datas", to name but a few, confirm. Its the most consistently excellent Trek season since the fourth. Unfortunately, its got some real stinkers - "Schisms" and "Realm of Fear" are not favorites of mine at all. Nevertheless, episodes like "Chain of Command", "Tapestry" and "The Chase" do prove that TNG could still be excellent, if it wanted to. Also, the darkest TNG season since the second - and definitely the most intrigue-filled one.
DS9 saw the Dominion War commensing. How wonderful, indeed - "His Way", "Times Orphan", "Far Beyond the Stars", and my personal favorite DS9 episode, "In the Pale Moonlight", signal this season's almost uncontrolable brilliance. Trully, a great season, and DS9's pinnacle to excellence.
Perhaps the best post-modern Trek season? Well, I gotta rewatch it again, to decide... But if I remember correctly, this is a winner!