As for me, don't really consider myself a trekky but still quite enjoy watching them....
Star Trek TOS - come on....it's rubbish, eh?? I mean, really, Kirk gets to snog an 'alien' female every other episode and Spock, well, let's not go there.
Star Trek TNG - The best, in my opinion - not as 'dark' as DS9 but on the other hand, characters that were more interesting, a great captain (come on, is there really any competition between Patrick Stewart and the others???) and innovative for its time.
DS9 - 'dark' as said, had a continuing story arc, which was refreshing, and new enemies, but still not really that dark and so much more could have been done with it.
Voyager - Great ideas, but immediately annoying captain (chew some lockets, love) and Harry Kim, wtf? Seven of Nine was when the shark was totally jumped, if you ask me.
Enterprise - Scott Bakula should have rested with Quantum Leap instead of going on to this. Early episodes promising but temporal cold war totally pointless and messed up the other franchises as well as turning viewers off in droves.
There is my completely unbiased and authoritative opinion...not.