Davidson: Clustering Key. Usually, the PK, but not always. However, be careful if you are not using the clustered index for the PK. In OLTP systems, it is prevalent that while logically you feel like you are going to fetch the row by a natural set of values, it often turns out that internally, the PK is used to fetch the rows.
In one system we worked on, changing the key on the invoice line item to cluster on the invoice ID was discussed. Logically, you feel like you fetch the rows by invoice, not one at a time. It was true for the one fetch, but the code was accessed many more times by the key as processing occurred.