Optimam partem elegit
Communion used before the Council of Trent for various Marian feasts; assigned to the Assumption in the 1570 Missal; and now assigned in the post-1970 books to OT 16C and the Common of Virgins.
Source of text is Luke 10:42.
Settings by composers
A longer part of the Gospel is set in Intravit Jesus (Giaches de Wert) SATTB
Text and translations
Optimam partem elegit sibi Maria
Den besten Teil hat sich Maria erwählt,
Mary has chosen for herself the best part