TOS - Hard to beat because they set the standard. All the characters since are modeled on the orginals. James T. Kirk rules!
TNG - Rough going the first three seasons...then the Borg came, and saved the series! Writing and acting got much better in season four. They did some great "issue" episodes that mirrored what TOS tried to do. Once Patrick Stewart got his space legs he made a great captain. Everybody raves about Troi, but Bev Crusher..YUM! Q was a hoot!
DS9 - The darkest and probably most controversial Trek series. People seem to either love it or hate it. I loved it. Not many happy endings and the characters were put into a good story cycle that spanned the seasons. Good move bringing Worf and O'Brien on board. The Odo/Quark rivalry reminded me of the Spock/Bones banter from the TOS. Really liked the idea of the Defiant, wish they'd explored that concept in full starship size. Ezri - Yum!
Voyager - Great concept, great characters, horrible execution after season two. They could have done SO much more with that show. Have a little more tension between the Starfleet/Maquis crew members, having to improvise and barter from people they meet, have the ship look a little more worse for wear each season. I would have brought them home at the end of season four and had them deal with everything in season five: court-martials for the Maquis, guinea pig experiments on Seven, determining the sentient status of the Doctor, and maybe a court-martial for Janeway for violating the Prime Directive in the Delta Quadrant. I'm sorry they got rid of Kes. She was a good character, they just didn't seem to know what to do with her.
Enterprise - very promising from what I've seen (not much unfortunately). I like the idea of not using the transporter all the time, and the "grappler" in place of the tractor beams! I wonder how they'll deal with the Suliban since they're never mentioned in the "future" Treks? T'Pol is OK but Hoshi is the one to watch!
After Enterprise ends its run they should hold off on another Trek series for several years. Give the franchise a chance to rest and sink in to another generation. Let some new writers and talent take over and then fire up another series five to ten years down the road.