Church Slavonic. Because it's a special liturgical language, but after some investigating likely to understand. Words full of theological meaning, beautiful sound (doesn't matter which variant). Definitely not Polish, nor even Serbian (although it's more acceptable as it's close to Church Slavonic).
But as for readings (especially Old Testament, but also lectures from the Apostle, possibly Gospel), I prefer vernacular. Because generally they're not poetry like hymns, and the their first purpose is teaching (of course, it's also intent of the hymns, but not in the first place, because they aim to enable people to pray and focus on Divine matters).
Of course, I love also Arabic, Syriac, old Greek languages... They have beautiful melody, and I suppose, the meaning and richness of the theological vocabulary is very similar to Church Slavonic (especially it[s the case of Greek, as CS was based on it).
I also really like when on the greatest feasts (Pascha, Nativity, Epiphany, Pentecost) some parts are chanted in other languages (that's practice of my parish). Then some parts in vernacular are for me OK. Also when we celebrate a feast of particular saint and one-two prayers are chanted in the language of this saint (e.g st. Sava - Serbian; st. John Damascene - Arabic; Chinese Martyrs - Chinese etc.)