I get a little giddy thinking about digging through AO3’s mountains of fics, because for me the fun part is the treasure hunt more than a single leaderboard. If you want the works with the most kudos or the most comments, the quickest path is to use AO3’s built-in sorting and filters: Browse Works → sort by 'kudos' or 'comments' and narrow by fandom, rating, or language. That shows you the cream-of-the-crop at whatever slice you choose. I tend to do this when I want to binge something with community momentum—there’s a comforting familiarity to seeing a mega-chapter epic with thousands of kudos and hundreds of comments.
Practically speaking, the pieces that sit at the very top are almost always long, multi-chapter epics in huge fandoms. Think major universes like 'Harry Potter', 'Sherlock', 'Supernatural', or 'Star Wars'—those fandoms produce the volume and audience that push works into the high-thousands of kudos and comment counts. You’ll also find that well-tagged, trope-friendly stories (enemies-to-lovers, found family, fix-its) get traction quickly. If you’re chasing the absolute highest numbers, filter for language and sort globally, but expect that the top results change slowly as older classics keep accumulating kudos.
If you want specific names, I usually cross-check the sorted AO3 results with community-curated lists on Tumblr, Reddit, or fan blogs—people love to make top-fic lists. Another trick: search AO3 with the URL parameters (add sort_column=kudos or sort_column=comments) to get a descending list. It’s not a single canonical hall-of-fame, but between AO3’s sort, fandom community lists, and a bit of patience, you’ll find the threads of fandom history stitched into those top works. I often end up saving a handful to my bookmarks and re-reading on slow Sundays—some of those highly-kudoed stories age so well.