omg i love this question!! so so many good ones but here are some of my favs in terms of characterization (there's a lot of good fics out there featuring rik but these are the ones that come to mind first + anything by adultery while they're still up 😭)

https://archiveofourown.org/works/48713869 (suspended in a sunbeam - JELLYPOP): so so so yummy and such a good ggr study i love this fic to bits

https://archiveofourown.org/works/49463902 (gold of white (mine or not?) - rocketsummer): another rlly good rik pov ggr study that i love w/ very potent atmosphere

https://archiveofourown.org/works/51575116 (if you can’t hang - chaste): rik’s characterization in iych eats so bad and i love how gyubrik's relationship develops here