Widdy correctly guessed that Flower Communion is celebrated by approximately 88% of Unitarian-Universalist congregations. This puts it well ahead of traditional Communion, as well as many other ceremonies found in UU circles.
Patrick Murfin suggested that chalice lighting is even more popular in UU congregations. The way the question was contextualized was specifically meant to limit it to irregularly-held larger ceremonies, such as communions, rather than elements that appear every Sunday. Otherwise, the correct answer could’ve been “listening to sermons,” “singing hymns,” or even “announcements” (all of which have ritualistic elements). Also, we don’t have actual data on chalice lighting percentages, while we do for Flower Communion and many other ceremonies.
That said, chalice lighting is an important, nearly universal, and uniquely UU ritual, and deserves mention. Thanks to Patrick for bringing it up.