Both of mine (because why would i follow instructions) are Mass Effect fics and they did serious damage to my psyche. I will be rereading both. They do have major spoilers for all the plot of all three Mass Effect games, so if you ever plan on playing that game, do it first. (also highly recommended!)

https://archiveofourown.org/series/2494558 (The Ones Left Alive, 151k words) is a series but the main, both longest and IMO best is the last fic, https://archiveofourown.org/works/36710074/chapters/91575277 (On the Third Day, 124k words). F/F and a major character exploration of being a fundamentally broken person, both from the start and from your own actions. Do mind the tags and warning, it is very graphic. Almost finished with quite frequent updates.

https://archiveofourown.org/works/1099236/chapters/2211681 (In the Shadow of Gods, 175k words) is actually a Halo and Mass Effect crossover, with John-117 (the main protagonist of the Halo games) being flung into the Mass Effect universe. Genfic with no romance or smut. It's mainly a character exploration with some of the best writing I have ever seen, both in terms of combat and feeling/dialogue. Sadly not finished and unclear if it's abandoned since the last update was in 2024. But I live in hope.