I loaded up the hacker news dataset in google bigquery, which turned out https://news.ycombinator.com/item?id=42057647 as the biggest at 9624 comments. Second is https://news.ycombinator.com/item?id=43208973 at 5349.

Could be this one, perhaps?

https://news.ycombinator.com/item?id=43208973 ("Zelensky leaves White House after angry meeting (bbc.com)")

5044 comments, or 5349 with "showdead"

I used a null query,

https://hn.algolia.com/?query=&sort=byPopularity