As for as Orthodox/Eastern Catholic chants go, I love the Cherubic hymn and I try to find as many arrangements of it as I can.
The Holy Friday Lamentations are extremely beautiful too, particularly the Greek melody, with extremely deep words. The first Byzantine chant I ever heard was Agni Parthene (under the moniker Feciora curata because it was a Romanian choir), and I'm pretty sure I teared up over it.
On the Roman end:
Te Deum, solemn tone.
Salve Regina, solemn tone.
Missa Orbis Factor
Le messe de nostre dame by Machaut
Holy God We Praise Thy Name
Ave Maris Stella
In spite of my Theological differences with the Latins, I believe that they have either given us, or inspired some of the most beautiful music dedicated to the worship of God. In addition to the Te Deum, the Ave Maris Stella list above is beautiful, particularly as arranged by Claudio Monteverdi in his Vespers. However, it is my opinion that the peak of music dedicated to God are the two Sacrea Symphoniae written by Giovani Gabrieli in the turn of the 17th Century. The last of his hymns, lost after his death and which he probably never heard performed, is one of my favorite. It is the Sonata con voce: Dulcis Jesu a 20. It was only discovered about 60 years ago in a castle in Germany, preserved there, it is believed, by one of his students. His music influenced the later great German composers, Roman Catholic and Lutheran, all the way up to Bach. Yet, in his home, Venice, he was quickly forgotten, his music overshadowed by the operatic excesses of those that followed him, including Monteverdi. I don’t believe that Gabrieli was simply a man. I believe, like his last name, he was an angel. I don’t know of any man who could have written what he did.
Of the later composers, the German Michael Preatorius is probably my favorite. He was a very prolific German (Lutheran) composer who is best known for his arrangement of French dance tunes (the Terpsichore), and the writing of his work the Syntagma Musicum; required reading for anyone trying to understand the music of that period. However, he also wrote over 1200 church hymns and arrangements, many in use by Lutherans and other Protestants to this day. Some have had to be toned down due to the passions raised by the Reformation, and later the 30 years war that devastated so much of Europe, but the music remains beautiful to this day. Like me in my younger days, Preatorius was a militantly Lutheran Christian that regretted not becoming a monk or a priest. This presented an interesting dichotomy given the rejection of holy orders by the Lutherans, and this tension and dichotomy are often sensed in his music.