I've seen my share of each of the incarnations, each with their own strengths and weaknesses. I really like the original series because frankly without that, we wouldn't have had the rest. That being said, it is a shame that the original series didn't have more time under its belt, but the long-term effect is still being noticed today.
TNG was next in line, and in my opinion, the one with the best amount of experimentation. DS9 was fine, gave you a different perspective on the ST universe. Voyager was o.k., but seemed to lack an overall purpose, other than them trying to get home. Enterprise... nice trying to run back to let us in on earlier history, but prequels are so 1999.