Sort by citations (https://dl.acm.org/action/doSearch?SeriesKey=cacm&sortBy=cit...) to see the most seminal ones.
I did that and found a paper I would rank highly: Hoare’s “An axiomatic basis for computer programming”[1].
I clicked through and saw that it was included in their 25th anniversary issue and that particular issue seems like an excellent place to start.[2]