Congrats to Philocrites, who quickly guessed today’s answer. The most commonly sung hymn in UU churches is “Go Now in Peace.” The reason for this is that it is the most popular children’s recessional, and therefore occurs in most congregations that have a separate Sunday School program. Interestingly, the words are often altered from those in the hymnal, Singing the Living Tradition. Whereas the final line exclaims “May the love of God go with you,” many churches actually sing the line as “May the spirit of love surround you.”
If we look at hymns that are actually sung as hymns, then the most common one is “Spirit of Life.” This is definitely a song that is not only widely appreciated as praxis, but also has made a theological impact on many singers. However, like all things UU, it also has its detractors, with a vocal minority who actively dislike it.