List zfs filesystems and sort them by the space used by their associated snapshots:
zfs list -o space | sort -k4 --human-numeric-sort
Example output:
root@nas[~]# zfs list -o space -r slowtank/backups | sort -k4 --human-numeric-sort
NAME AVAIL USED USEDSNAP USEDDS USEDREFRESERV USEDCHILD
slowtank/backups 2.80T 23.3T 11.6K 145K 0 23.3T
slowtank/backups/timemachine_jt_personal_mbp 170G 86.4G 1.60M 86.4G 0 0
slowtank/backups/database_backups 2.80T 648G 2.86M 648G 0 0
slowtank/backups/jtmbp_old 2.80T 338G 3.33M 338G 0 0
slowtank/backups/borg 2.80T 3.05T 7.29M 3.05T 0 0
slowtank/backups/restic 2.80T 11.2T 13.2M 353M 0 11.2T
slowtank/backups/mail 2.80T 23.5G 662M 22.8G 0 0
slowtank/backups/longhorn-backups 2.80T 183G 3.59G 179G 0 0
slowtank/backups/restic/restic_data_justyns 2.80T 1.18T 6.98G 1.18T 0 0
slowtank/backups/timemachine 159G 341G 21.3G 319G 0 0
slowtank/backups/oldtank_backups 2.80T 1.45T 24.1G 1.43T 0 0
slowtank/backups/servers 2.80T 593G 145G 448G 0 0
slowtank/backups/justyns 2.80T 2.26T 274G 1.99T 0 0
slowtank/backups/archived_hdds 2.80T 2.80T 302G 2.50T 0 0
slowtank/backups/restic/restic_data_servers 2.80T 9.73T 1.66T 8.07T 0 0
From the above list, it is then easy to figure out slowtank/backups/restic/restic_data_servers uses the most snapshot space (1.66TB). Snapshots can then be listed for that filesystem by using a command like:
zfs list -t snapshot -r <zfs filesystem>