DS9 was the best! I loved the Dominon War: mining the worm hole, defending the station from an all out attack, assassinating a Romulan Senator and framing it on the Dominon to get the Romulans to join the war as Federation allies. That's suspense!
Voyager is 2nd, started badly and came back. I liked the fact that it had cool technology: EMH, Multi-vector starship that split to become 3 ships, slit stream warp and not to mention Jeri Ryan! I liked the ending even though many people hated it, but wish it showed more celebration of their homecoming
TNG is 3rd, I used to love it, now as I catch the reruns, its boring to me
Enterprise 4th: I watched 5 episodes but it got boring, I tried to image that it was before Kirk's timeline but it just didn't happen. Besides, I don't want to see the past, I want to see the future, and what it holds for the Federation.
TOS: Never saw more than 2 episodes, just felt like all Kirk was trying to do was to get laid, plus the way he talked, that......annoyed......me...
Just my 2 cents.