Pretty please?
Pergolessi – Gloria (This stands alone)
Bach - St. Matthew Passion (This is of course a Mass – Bach Mass in B minor)
Vivaldi’s Gloria.
Pergolesi, Stabat Mater, the first movement. This is the single most beautiful baroque choral movement. Anyone who’s heard it, will know what I mean. (is this what you meant, dgryphon?)
Vivaldi is good, too, as was mentioned.
Bach - St John Passion…much darker than the Matthew passion.
Halleluja - Messiah - GF Händel
Ooooh, I sang in this piece a few years ago! It really is wonderful. 
And you really can’t go wrong with anything from Handel’s Messiah.
Bach: Jesu, Joy of Man’s Desiring
Anerio: Requiem
Victoria: Domine, non sum dignus
Bach - “Nun ist das Heil und die Kraft,” the only remaining movement from his Cantata BWV 50.
Bach’s Magnificat.
If it ain’t baroque, it don’t need fixin’
Sorry
Purcell: The Fairy Queen - Hark! the ech’ing air a triumph sings. (I love the word painting in this one!)
Getting a bit out of Baroque and into the Renaissance/Late Middle Ages (if you don’t mind :))…
Orlando di Lasso: Motet - Tristis est anima mea
Claudin de Sermisy: Chanson - Tant que vivray
John Dowland: Air - Flow, my tears
Händel’s Messiah is very, very, very, very, very, very good 
Please keep in mind that different recordings can be very different. Some recordings may suit your taste more than other, and you may be unlucky enough to pick up the one “bad” recording of a piece you might otherwise like. If you are able to borrow music (library?) please try 2-3 recordings before you decide if you like it or not.
Bach’s *Cantata 140: Wachet Auf *
Michael Praetorius’ motet “Erhalt uns, Herr bei deinem Wort” is a really wonderful early Baroque piece. Definitely one of my favs.