Star Trek: DS9 was the best series on realism in that arena.
I guess you are limiting “the arena” to the Star Trek universe?
However, DS-9 was Trek’s response to Babylon 5, which was still far superior (though I rather liked DS-9 most of the time, too.)
Quite true. At its best, DS9 had a grubby realism that featured conflicting personalities, motives and aims at cross purposes, and surprising plot developments. In contrast, Star Trek and Star Trek Next Gen usually had at their heart a smug rectitude about their main characters and the Federation that limited dramatic potential.