Man, that's a pairing that sends you down some specific rabbit holes. Honestly, I gave up on trying to find a single "best" platform for it a while ago. A lot depends on what flavor you're hunting—pure smut, slow-burn romance, or alternative universe stuff where their dynamic is completely reimagined.
FF.net used to be the old guard, and there's a ton of volume there, especially older works from when the fandom was first speculating. The quality varies wildly, though, and the tagging system is basically non-existent. You'll be sifting through pages of unrelated Naruto/Hinata fics or Boruto-centric genfic just to find one. Lately, I've had better luck on Archive of Our Own. The filtering is a lifesaver; you can exclude the pairings you don't want and really dig into the specific Boruto & Hinata (Platonic or Romantic) or Boruto/Hinata tags. The writing quality tends to be higher on average, in my experience, but the pool for this specific ship is smaller.
Don't overlook smaller, fandom-specific forums or Discord servers either. Some of the most interesting takes on their relationship—especially the more psychological, forbidden-fruit angles—pop up in those tight-knit communities where writers aren't as worried about mainstream appeal.