I use the Western Digital 6TB WD Red Pro NAS Internal Hard Drive - 7200 RPM Class, SATA 6 Gb/s, CMR, 256 MB Cache, 3.5" (WD6003FFBX) drives in my FreeNAS. These fast and reliable CMR drives that are my favorite for this role.

Always check the model numbers you are buying to make sure they use CMR, not SMR storage.