Honestly, you're asking about a pairing that thrives outside the obvious spots. The 'Whitty x Ruv' tag isn't going to be well-populated on the big archive sites, it's a deep cut from the 'Friday Night Funkin'' mod scene. Most of the solid stories I've found are scattered across smaller, more niche platforms or tucked into Discord servers dedicated to the specific creators' work.
You really have to dig into where the mod fans hang out. Tumblr tags are surprisingly active, but the format means stories are often broken into threads and reblogs, which can be a pain to read in order. I've had more luck on sites like Toyhou.se, where artists and writers sometimes post ficlets alongside their character profiles. The quality varies wildly, though—some are just fluffy headcanons, others have genuinely interesting takes on their post-canon dynamic.
AO3 does have some, but you need to use the right combination of tags like 'Friday Night Funkin' (Video Game), 'Whitty (Friday Night Funkin')', and 'Ruv (Friday Night Funkin')'. Filtering by kudos helps, but a lot of the best ones are buried because the fandom moves fast. There's this one author who writes them in-character perfectly, capturing Ruv's stoicism and Whitty's pent-up frustration, but they deleted their account last year. It's that kind of pairing—ephemeral.
My personal advice? Find the fan artists you like for the ship and check their social media links. They often collaborate with or recommend fic writers. It's less about a single 'best' repository and more about following the breadcrumbs through the community.