I think because I have been watching it since when it first started in the 1960's (yes I am that old).
I have found for the most part they all did,( for their time) a great job.You have to also remember, they could only represent women, (for that time) way back in the 60's....in certain parts.
So for the ones they did have on 'the bridge', that was Big!
Women had very small roles way back then, and I feel Star Trek made way for women to not only be seen, but heard.
I can't say any have fallen short after watching it for the past 50 years!
If anything it had jumped in leeps, compared to other t.v. shows.
Voyager is the obvious answer because it had a female captain, but the show also had three other very strong female leads. DS9 also had very strong female characters, not only in the main cast, but with guests as well. The Original Series really did break boundaries for both women and black people by having Uhrura on the bridge, but all the other women on the show were mainly just fodder for Kirk love stories.
I think TNG was weaker at portraying female characters, because the women are al in stereotypical caring roles (doctor, therapist), and I found Troi and Crusher could be Whiney. They tried to make something of Yar, but made her too volatile and defensive. As for Enterprise, I found Yoshi's character disappointing and T'Pol too sexualised.