I've always been partial to the new uniforms that started with First Contact. The black and gray looks more like what a military uniform would look like, I think. After that I'd have to go with the TNG uniforms from season 3-7.
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The interesting thing about the history of the uni's, is how military-like they should look. Gene Roddenberry was not for the idea that Starfleet represented the military - ST:TMP certainly straddles a feeling of officers & scientists in the look of the uniforms. Roddenberry was put more in the background from then on, though(due to his constant demanding on set & off), and the look was changed by Nicholas Meyer, who felt Starfleet was an intergalactic navy, like it or not.
I was in favour of the change from Generations to First Contact, as those NG uni's looked designed for the brighter light of TV; just like the original series.
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I like the uniforms in the 3rd or 4th season of the Next Generation, after they got the collars and 86'd the back ground character man skirts. (which I don't think they actually did much after the first few episodes of the 1st season.)
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I liked the red II-IV uniforms. It united the crew a bit more from the different colours of the series. I also liked the section that opened from the shoulder because it was used in a really expressive way in the films.
Remember McCoy ripping the section off and going to get a "cup of black coffee" in TUC or Kirk striding down the corridor doing up the uniform before facing Khan again. Just minor things of course but they all help the films in different ways.