This one is easier than the last two.
1.DS9 easily takes this one. It had the best final year owing a great deal to all the previous seasons of setting up threads and character arcs that finally had no choice but to come to some sort of resolution as the series was ending. A major reason I find this season to be the show's best is because of the Final Chapter Saga where everything came together for a thrillingly unpredictable ten week adventure.
I loved how threads came together in interesting, unpredictable yet satisfying ways from Rom becoming the Nagus, Gowron being deposed and Martok the new Klingon Chancellor, the Breen joining up with the Dominion, Winn forging an alliance with Dukat, the Defiant being destroyed, San Francisco attacked, Section 31 being behind the Founder malady, Odo becoming sick, Kira donning a SF uniform and teaching the Cardassians resistance tactics, the parallels between Bajor/Cardassia and Cardassia/Dominion, Garak's homecoming, seeing Mila again etc.
I have fond memories of those months of April/May/June in '99 and I loved the weekly promos with "The End of a Legend Has Begun".
I know some find issues with how certain things resolved but I tend to not dwell on what might have been and focus on what was and overall I was satisfied. I thought the Worf/Ezri/Bashir stuff was overdone and Extreme Measures was a pointless momentum-killer but overall the final season was ambitious, engrossing and satisfying.
And I also enjoyed episodes like Badda Bing Badda Bang, Take Me Out to the Holosuites, Emperor's New Cloak, Inter Arma Silent Leges.
TNG on the otherhand was running on fumes. The efforts to prepare VOY, write for DS9 and Generations divided attention coupled with obvious burn out made this a disappointing season. In years past the mediocre episode was the exception this year it was the norm--Force of Nature, Masks, Eye of the Beholder, Emergence, Homeward etc.
VOY-well what can I say it was VOY. It had its decent episodes, mediocre ones but I felt the Unimatrix Zero rebellion that started off so promising in Part I was botched, the final stretch of episodes focused on the mundane and pointless and Endgame was disappointing.