I am running a NAS that needs to connect to a server (the NAS isn’t powerful enough). I also need to connect my NAS to a Windows, Mac, and Linux device (Linux being the most important, then Mac, then Windows). Out of SMB, FTP, and NFS, which one would be the best, quickest, and most secure for my situation? My NAS supports multiple sharing protocols, but I don’t want to deal with mixed up permissions and conflicts later on.
FTP, unless you are using it over TLS (FTPS) transfers authentication information in the clear and transfers files in the clear.
SMB and NFS both can, depending upon implementation and configuration, be set up to do encrypted transport. Note that it is possible to have either doing secure authentication but not secure transport of data.
From a quick search, it looks like at least some Synology NASes can do secure NFS:
https://kb.synology.com/en-us/DSM/tutorial/what_can_i_do_to_encrypt_data_transmission_when_using_nfs
As well as SMB:
https://kb.synology.com/en-us/DSM/help/SMBService/smbservice_smb_settings?version=7
As well as FTPS:
https://kb.synology.com/en-global/DSM/help/DSM/AdminCenter/file_ftp_setting?version=7