The servant of the church
Whether Phoebe's title names an office of deaconess or a general service.
Romans 16:1-2
1I commend to you Phoebe, our sister, who is a servant of the assembly that is at Cenchreae,
2that you receive her in the Lord, in a way worthy of the saints, and that you assist her in whatever matter she may need from you, for she herself also has been a helper of many, and of my own self.
What each tradition says
Rabbinic / Midrashic
No sourced position found. Checked the Jewish Encyclopedia (jewishencyclopedia.com) by site-targeted searches for 'Phoebe', 'deaconess' and 'Romans 16': the JE has no article on Phoebe or the early Christian diaconate, and the JE articles fetched for this batch (Baptism, Agape) do not mention Romans 16:1. No citable Jewish statement on Phoebe's title found.
Patristic
Chrysostom identifies Phoebe as a deaconess of the church at Cenchrea, noting the A.V. renders it “servant.” He says Paul gives her dignity in many ways, mentioning her before all the rest and calling her sister.
John Chrysostom, Homilies on the Epistle to the Romans, Homily XXX (NPNF ser. 1, vol. 11, trans. J. B. Morris & W. H. Simcox) — Homily XXX, on Romans 16:1 ('I commend unto you Phebe... a deaconess')
Catholic
Haydock takes Phoebe to be in the ministry, employed in the ministry. He explains that women called diaconissae used to do this: privately instructing catechumens, assisting particularly at the baptizing of women, and distributing charities.
George Leo Haydock, George Haydock's Catholic Bible Commentary (1859) — Note on Romans 16:1 ('Phoebe... in the ministry')
Orthodox
The Council of Chalcedon's Canon XV states that a woman shall not receive the laying on of hands as a deaconess under forty years of age. Even then, she may receive it only after a searching examination.
The Council of Chalcedon (451), The Seven Ecumenical Councils, Canon XV (NPNF ser. 2, vol. 14, ed. Schaff & Wace, trans. H. R. Percival) — Canon XV of Chalcedon — on the laying on of hands for a deaconess
Reformed / Protestant
Calvin says Paul first commends Phoebe, to whom he gave this Epistle to be brought, and commends her for her office. She performed a most honorable and most holy function in the Church.
John Calvin, Commentary on the Epistle of Paul the Apostle to the Romans (trans. John Owen; Calvin Translation Society) — Commentary on Romans 16:1 ('Phoebe our sister, which is a servant of the church')
Evangelical
Wesley notes that the Greek word for “servant” here is “deaconess.” In the apostolic age, he says, grave and pious women were appointed deaconesses in every church; their office was not public teaching but visiting the sick, especially women, and ministering to their temporal and spiritual needs.
John Wesley, Explanatory Notes upon the New Testament — Notes on Romans 16:1 ('I commend unto you Phebe')