My sample isn't particularly large, but RHEL and SLES still appear to be the popular choices. CentOS is also sometimes used. Maybe someone else has a better overview?
In my experience, a few systems (tens) I came across used centOS on the compute nodes, and RHEL on service/login nodes. Sometimes I would come across some custom thing (e.g. one of the big systems in China uses Kylin). But the RHEL and/or CentOS choice is by far the most popular.