My favorite is far and away the TWOK-TUC uniforms. As others have said, they look the most to me like a true military uniform. Though, to be honest, I didn't really like it when Nick Meyer took them one step farther toward military authenticity by adding the ribbons and medals to several of the uniforms. But, overall, I think it's the most authentic looking, as well as the best in appearance. I agree it seems more like a dress uniform than a duty uniform, but we see that there are variants for those working in engineering, those on landing party and combat duty, etc.
My second-favorite would be the 3rd season and later TNG uniforms.
Least favorite probably goes to the late DS9/First Contact uniforms. Why? Because they are so lacking in color. Star Trek, in all other incarnations, has always had bright, colorful uniforms. That might not match our modern military, but it has been distinct for Star Trek, and I think it's looked good. Although the First Contact uniforms certainly look well-constructed and don't look bad on the actors, I miss the colors. Same with the early DS9/Voyager uniforms. The little bit of color isn't enough for me.