Depends on what you mean by 'top-rated' honestly. Straight up kudos count on AO3 is a decent metric, but the algorithm there can bury some older gems. For Lee/Gaara, I'd start by filtering the ship tag on AO3 by 'Kudos' and maybe 'Bookmarks'. The classics from the mid-2000s on FF.net have a different feel—lots of epic battle-to-lovers stuff—but finding the highly favored ones there means wading through pages sorted by reviews. Sometimes a story with 800 reviews from 2010 has more cultural weight than a newer one with higher stats.
Don't sleep on curated collections or rec lists either. I found my absolute favorite, 'Gravel and Grace', through a Tumblr post that was just someone screaming about it. That story has a relatively low kudos count but the prose is insane; it's all about Gaara's internal monologue and Lee's relentless optimism clashing in the best way. The 'top' stories aren't always the ones with the biggest numbers, especially for a ship that peaked in activity a while back.
My personal method? I look for authors who tag meticulously. If someone uses tags like 'character study', 'post-war', 'slow burn', and 'canon-compliant', the quality tends to be higher, even if the hits aren't astronomical. The real treasure is in the comments section of those fics, where people are still passionately discussing them years later.