Another vote for the First Contact/late DS9 unis. To me, they beat out all the other uniforms in both visual appeal and
functionality (at least as far as we can assess the functionality of fictional uniforms used by a fictional organization hundreds of years in the future
). It looks very professional, and very sharp. And I do think that certain characters of the fairer sex (*cough*Ezri*cough*) do look particularly nice in it, as well. To quote Quark: "I love a woman in uniform!"
I do like both the early DS9/Voyager uniform and the TNG 3-7 uniform, also (although I never liked how stiff the collar looked on the latter). As for the 23rd century, the TWOK-TUC one isn't bad, but as others in this thread have said, it never looked right as a duty
uniform. It had an almost civil war era flavor, with the stripes on the pants, the puffy white collar, and the decorations on the strap near the right shoulder. It would have been good as a dress uniform, with something a little more simple (and that didn't look so movement restricting) as the duty uniform.
The Enterprise uniform was certainly good in terms of looking right for it's time and setting. It still had a modern, NASA-esque quality to it, yet it looked just different enough to pass as something worn by Starfleet officers. And it does win for functionality, as well (actually, a couple of cargo pockets is something I've always thought would be a cool addition to the FC/DS9 black/gray uniform).
The worst uniforms are probably those ridiculous, no-collar, "stretched out one-piece" TNG season 1-2 ones, and those colorless, uber 70's, "PJs... in spaaaaaace"
things from TMP.
The original TOS TV show uniform style is ok... it does look really dated to me, but it's not too bad. The main thing I didn't like about these was the low-cut top/miniskirt combo that every female officer was seen in past "The Cage". This is the exact opposite of the FC/late DS9 unis when it comes to female characters; it looked completely unprofessional and made no sense for a show set so far into the future. I know, I know... it was a result of when the show was made,