The best RAID for a NAS depends on how many bays it has and how big the drives are. As a rule: mirror for two bays, RAID 6 or RAIDZ2 for four or more large drives. Size your exact NAS in the RAID calculator.
By number of bays
Two-bay NAS: use a mirror (RAID 1) — one drive of usable capacity, survives one failure, trivial to rebuild. Four-bay: RAID 6 or RAIDZ2 is the sweet spot on large drives (two-drive resilience with good capacity); RAID 5/RAIDZ1 only if drives are small and you have a backup.
Six-bay and up: RAID 6 or RAIDZ2 for capacity; RAID 10 if the NAS serves write-heavy workloads like databases or busy iSCSI. Avoid wide single-parity arrays on big drives.
By drive size
Drive size matters as much as bay count. On 8 TB+ nearline drives, single-parity RAID 5/RAIDZ1 carries real rebuild risk (see is RAID 5 dead?), so dual parity is strongly preferred. On small SSDs the risk is far lower and single parity can be acceptable.
ZFS-based NAS units (TrueNAS, and many others) add checksums and self-healing, so RAIDZ2 is the popular default — just remember a RAIDZ vdev has the IOPS of one drive, so use mirrors if the NAS needs high random IOPS.
Don't forget the backup
Whatever RAID you choose, the NAS still needs a backup — RAID is not a backup. A second NAS, cloud object storage with immutability, or LTO tape gives you the off-site, ransomware-resistant copy. See RAID is not a backup.
Use the calculator to compare a couple of layouts on your drives, then talk to us about the NAS or array and the backup tier to go with it.