Here's my view on it. TOS was the best, period. It was a sci-fi drama, which had a perfect combination of action and adventure. They would explore, come into conflict, explore some more, lose a few people to a rebellion, then continue exploring, etc.
When Fox created the next incarnations, they built upon the existing franchise as 80(?) years after TOS. The galaxy was already known about, and therefor not much adventure-seeking was needed. TNG/DS9 was about conflict with existing and new species. The series were action-based dramas, with exception to the first season, to solidify character and atmosphere.
And then, for some ****ed up reason, Paramount created UPN, and after that, they took their creation away from Fox networks, and hosted it under their own network. Big mistake. After ~30 years of an idea of what the galaxy was, it was all thrown out the window. UPN re-invented the wheel, and there were idea conflicts EVERYWHERE! For example, the Klingons were mild mannered, easy going warriors?? That doesnt make any god damn sense! And this whole thing about "traveling" to the other side of the galaxy, where the federation has never been before, but somehow everybody knows everybody else.
I never saw Enterprise, but I do recall someone from the Enterprise or Voyager series admit that the two series were not what Gene Roddenberry (sp?) wanted Star Trek to be. As far as I'm concerned, Star Trek will not be Star Trek without the original directors and producers from the previous series.