I think that the Annunciation appears more often than anything else in the Divine Comedy. Here are just two examples…

Purgatory Canto 10

The angel who came down to earth

with tidings of the peace so many years

wept for in vain, that op’d the heavenly gates

from their long interdict, before us seem’d,

in a sweet act, so sculptured to the life,

he look’d no silent image. One had sworn

he had said, “Hail!” for she was imaged there,

by whom the key did open to God’s love,

and in her act as sensibly impress that word,

“Behold the handmaid of the Lord,”

as figure seal’d on wax. HC

And again in Paradise Canto 23

… Angelic Love

I am, who thus with hovering flight enwheel

the lofty rapture from that womb inspired,

where our desire did dwell: and round thee so,

Lady of Heaven! will hover: long as thou

Thy son shalt follow, and diviner joy

shall from thy presence gild the highest sphere.” HC

As the angel finishes, all Heaven echoes Mary’s name and sings the Regina Coeli.

Later in Canto 32 Gabriel sings again before the Blessed Virgin,

“Ave, Maria, Gratia Plena.”