The hashtag or phrase was used more than nine million times in 2015.
Third on the list was #MarriageEquality to mark same-sex marriage being legalised nationally in the US and Ireland this year.
Campaign: gay marriage supporters in Dublin react to results on the referendum for same-sex marriage in May
EPA
Other popular hashtags included #RefugeesWelcome to mark the debate around refugees fleeing war in the Middle East for Europe; #IStandWithAhmed in support of Texas schoolboy who was arrested when his home-made digital clock was mistaken for a bomb; and #TheDress, which divided the world over whether it was blue and black or white and gold.
Clock gaffe: Twitter users posted support for Texan schoolboy Ahmed Mohamed after he was wrongly accused of having a bomb
EPA
Twitter's global users grew to 320 million this year, with around 15 million in the UK.
Lewis Wiltshire, Twitter's director of media partnerships, said: "2015 has been another phenomenal year on Twitter. Across the world people have come together to celebrate, support one another, to show solidarity and of course to laugh."