I will take a stab at this since I am new here.
1. Star Trek-
This is the best. The original. Maybe one of the three best television shows in the history of the medium. It was way ahead of its time (which none of the other series can claim) and it broke entirely new ground on many different levels. It took enormous risks, was extremely intelligent on it's writing and story structure, and the characters (and actors) are cultural icons.
2. Star Trek: Deep Space Nine
A Trek that dared to be different. Between the space station setting, the background of the Bajorans spiritual life vs the Cardassians occupation, and the anthology/serialized storytelling format, DS9 was sadly the last time Trek dared to be unique. The ensemble cast was brilliant, and like TOS the relationships and development of the characters and the exploration of the human condition was at the forefront of every story.
3. Star Trek: The Next Generation
Moments of brilliance and an OUTSTANDING and talented cast couldn't quite put this over DS9. Unfortunately, Roddenberry's influence over this show was at the height of his delusions of grandeur about being a visionary and prophet , and he sacrificed good drama as a result. Unfortunately TNG was bloodless and pretentious as often as it was interesting. And, it was rarely FUN. Still, an enjoyable series, but not the best.
4. Star Trek: Voyager
I can't, despite valiant efforts, finish a rewatch of this show. It's a hollow re-hash of TNG with weaker actors. It was more fun than TNG, but I can never shake the feeling that I'm watching an entire tv series based on Star Trek: Insurrection. It's fun and fluffy, but more often than not it's also irrelevant and completely un engaging. Largely a missed opportunity.
5. Star Trek Enterprise
Speaking of missed opportunities! Rather than being a prequel show about the pioneering beginnings of FTL space travel, this is TNG with a worse ship. The characters were poorly conceived, turning out either terribly derivative (T'Pol, Tucker) or terribly boring (Travis, Hoshi). This is the only series I couldn't maintain enough interest in to bother finishing.