I recently purchased Sins of a Solar Empire: Rebellion via the Steam sale, and while the base game itself is great on its own, I found myself really blow away by how good some of the mods for it are. In particular, the Star Trek: Armada 3 mod has an astonishing level of polish and attention to detail put into it; in fact, it's probably the closest any game has come to actually simulating the experience of controlling a battle from the original shows, even out shining some of the official Star Trek games, like Star Trek Online.
In the past, I was always leery of playing game mods, for a variety of reasons too numerous and involved to get into here. However, after finding myself enjoying the ST:A 3 mod as much as I have been, I've begun to wonder if I've been missing out.
As such, I made this thread to see if people here had any recommendations for similar mods or just would like to gush about some of their own favorite ones. Are there any other examples of mods that take already good games and make them amazing? Or even ones that make games that are otherwise mediocre and turn them into something actually worth playing?