OMG, it's difficult for me to choose one the best ST series. I love TOS as much as TNG, I love TNG as much as DS9. I haven't watched VGR and I've seen only 1st season of ENT.
What I can say - ENT was a little bit worse than other series which I'd watched.
TOS is classic, it's special, other, it's got a spirit and I can't look at this in the same way as I look at TNG or DS9. I love TOS - this series made me Trekkie.
TNG and DS9 are great and I can't tell which is better.