I'm assuming you're asking about the pair from 'Eureka Seven', right? It's been a minute since I was deep in that fandom, but I was a total Renton x Eureka shipper back in the day. Finding dedicated spaces for them now can be tough because the anime's not exactly new, and the fandom's activity has cooled down a lot compared to its peak. Your absolute best bet is going to be Archive of Our Own (AO3). That's where most of the organized, tagged fic for older series tends to migrate to or get preserved. You can search by the 'Renton Thurston/Eureka' relationship tag and filter by kudos or comments to see what's still considered popular.
Don't sleep on FanFiction.Net either, even though it feels ancient. There's a ton of legacy content there from the mid-2000s when the show was airing. The search function is garbage, so you'll have to navigate to the Eureka Seven category and just start scrolling. The writing quality can be super hit-or-miss because a lot of it is from people's teenage years, but some of those older stories have a certain nostalgic charm, and you might find a few long-completed epics that were huge back then. I vaguely remember one called 'Falling Slowly' that everyone used to rec. Could still be up.
You might get lucky on some niche anime-specific forums or old GeoCities-style fansites if they're still miraculously online, but that's a real deep dive. Honestly, for a pairing from a series that's this old, 'popular' often just means 'the ones that are still readable and have a decent word count.' The engagement metrics won't be like for a current show, so sort by bookmarks on AO3 instead of just hits. That usually surfaces the stuff that left a lasting impression on the few of us still poking around in that tag.