I feel like the search for the 'top' stories in this category is a bit of a red herring. Rengoku's a popular character, so the 'most kudos' or 'most bookmarks' list on AO3 is predictable and huge. It's often dominated by the same authors who write for multiple fandoms. Instead, I look for the weird, specific tags that hook me. Stuff like 'established relationship,' 'cohabitation,' or 'post-Mugen Train fix-it, but Rengoku is alive and has chronic pain.' Those tags filter for a certain kind of writer who's thinking beyond the usual angsty death-replacement scenarios.
A story that's stuck with me wasn't even a major hit. It was a slice-of-life piece where the reader character worked at a Wisteria House and Rengoku was a frequent, loud guest recovering from missions. The dynamic wasn't romantic at first; it was all about him disrupting her orderly routine with his overwhelming presence, and her slowly realizing his boisterousness was a form of caring. You find those gems by ignoring the 'top' lists and diving into the tag wrangling.