Always my favorite, from Bones: "In this galaxy, there's a mathematical probability of three million Earth-type planets. And in all of the universe, three million million galaxies like this. And in all of that and perhaps more, only one of each of us. Don't destroy the one named Kirk."
For me, too. And De Kelley's delivery of this speech was just sublime.
Another favorite, from Charlie X, when Kirk tells Charlie: "There are a million things you can have in this universe and a million things that you can't have. It's no fun facing that, but that's the way things are!"