I'm sort of surprised it hasn't been mentioned already, but my favorite (finding the Latin is giving me only sheet music):
Come, O Creator Spirit blest,
And in our souls take up Thy rest;
Come, with Thy grace and heavenly aid,
To fill the hearts which Thou hast made.
Great Comforter, to Thee we cry;
O highest gift of God most high,
O Fount of life, O Fire of love,
And sweet anointing from above!
The sacred sevenfold grace is Thine,
Dread finger of the hand divine;
The promise of the Father Thou,
Who dost the tongue with power endow.
Kindle our senses from above,
And make our hearts o’erflow with love;
With patience firm, and virtue high,
The weakness of our flesh supply.
Far from us drive the foe we dread,
And grant us Thy true peace instead;
So shall we not, with Thee for guide,
Turn from the path of life aside.
O may Thy grace on us bestow
The Father and the Son to know,
And evermore to hold confessed
Thyself of each the Spirit blest.