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>