I have trouble with these kinds of questions, as it always comes down to two. I do strongly feel DS9 was the best Star Trek series. The writing and character development were truly exceptional.
But at the same time, I love TOS. I may not find it better than DS9, but there is a sense of comfort and relaxation I get watching those episodes I don't get from any other series. Perhaps it is because I grew up with those characters and can't remember a time not knowing who they were. The series has tons of sentimental attachment due to that, plus it also had some really amazing episodes.
Since this is for my favorite series, I'm going to give the edge slightly to TOS. Watching Captain Kirk and his crew trek across space never gets old for me.