SMB, FTP, or NFS for NAS + server?

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...