The 'best' for link/urbosa really depends on what you're hunting for, because the ship doesn't have a massive, centralized hub. If you want sheer volume and decent filtering, Archive of Our Own is the obvious starting point. You can tag for both characters and the pairing, then sort by kudos or bookmarks. I've found some truly epic, novel-length adventures there that treat Urbosa with the respect she deserves, making her more than just a plot device for Link. They explore the potential of that Champion-era camaraderie and what-ifs.
That said, don't sleep on dedicated Zelda fanfiction forums or smaller sites, even if they feel a bit old-school. I stumbled on a fantastic, ongoing slow-burn on a forum called Hyrule Castle Archives, of all places, that's all about political maneuvering and cultural exchange between the Gerudo and Hyrule, with the relationship developing very organically. It's got a depth you sometimes miss in the faster-paced stuff on bigger platforms. The downside is discovery is harder; you're digging through threads.
My personal gripe with just relying on tags is that it misses the crossovers. One of my favorite fics is actually a 'Breath of the Wild' meets 'Horizon Zero Dawn' crossover where Aloy and Link meet, and Urbosa's spirit plays a guiding role—it's tagged under both games but not primarily as Link/Urbosa, so you'd only find it by accident or deep diving. Sometimes the best takes on their dynamic come from stories where it's not the central, flagged romance but a significant element in a bigger plot. Tumblr blogs with specific recommendation tags can be weirdly good for surfacing those hidden, nuanced pieces.