The Holy Grail was a Christian artifact originally possessed by Jesus Christ, therefore extremely valued by Christians. The term "Holy Grail" was often used to refer to something that was extremely valuable or prized.
Upon learning about the Borg's desire for the Omega molecule, Kathryn Janeway called it the Borg's Holy Grail. (VOY: "The Omega Directive")
Doctor Julian Bashir claimed that the Holy Grail of organ replacement would be to come up with a way to inject the patient with undifferentiated tissue that could become whatever organ is needed. (DS9: "When It Rains...")