This is always a difficult question for me to answer. At the time of TNG's run, I loved the show. DS9 gradually edged it out as my favorite but I still really enjoyed the show. What makes answering the question hard now is that I can barely bring myself to rewatch a TNG episode unless it's a stand out episode like Yesterday's Enterprise, Chain of Command, or maybe Best of Both Worlds. I rewatch DS9 episodes regularly, even episodes that some would consider to just be filler, like the Ferengi stuff. If I had to rank overall the frequency in which I rewatch Trek it would be:
DS9
ENT
TOS
TNG
VOY