1. The Next Generation (Season 3 and beyond)
Whether you look up the behind-the-scenes drama or just watch the show, it is apparent that Star Trek: The Next Generation had a bumpy first few years. Although I am inclined to be more forgiving to the first couple of seasons than many people, something that decidedly improved in the third season was the uniforms.
The change was not drastic, and the overall look from afar is the same as before, but new costume designer Robert Blackman (whose name we've heard a few times before on this list) managed to tweak the original designs by William Ware Theiss (of TOS design fame) to a point of perfection, in my opinion.
The Season 3 uniforms preserve the asymmetrical color pattern of Theiss's TNG design while making them a bit more loose-fitting (to relieve the back pain Patrick Stewart had been experiencing) and adding collars. These slightly baggier uniforms are not, perhaps, as sleek as the Theiss designs, but they look far more comfortable, and the collars add a needed level of sophistication.
The reason why these uniforms are Number 1 on my list is not just that they're worn by Commander Riker, of course. A lot of it is that they manage to strike a perfect balance between simplicity and detail. The asymmetrical color pattern is striking and ensures that most of the top looks like a division color. And the collars, by having a dip down in front, stand out as not just being turtlenecks.
Conclusion
Of course, this list does not cover every major Starfleet uniform from every series—much less they myriad of variants, dress uniforms, etc. Honestly, I think the uniform designs are pretty consistently good across all of the different Star Trek series. These just stand out as some of the most quintessentially "Star Trek" to me.