I hold TOS/TNG/DS9 in roughly the same high regard. I really can't choose, and each has a lot to offer. I tend to collectively consider this grouping my favorite show ever.
I will say that when I watch DS9, I feel immersed in the Star Trek universe in a way the other shows don't compare to. The serial storytelling and richly drawn supporting characters really make you feel like you are there. because of the way the story is told, the events have more weight. You really feel in DS9 that this fictional universe could be a real place.
Well, that saves me from a making a statement in this topic, since it's basicly the same as mine. I love TOS/TNG/DS9, but I'll always be a Niner first and a Trekkie later.