This question is not very easy to answer. I was able to find some information but not an exact answer. Some topics or hashtags that have been very popular include 32.1 million tweets for #WorldCupFinal, 78 million tweets for the MTV Video Music Awards at #vote5sos, 4.2 million tweets for the #IceBucketChallenge, 3.5 million tweets for both #JeSuisCharlie and #Ferguson. The #JeSuisCharlie started trending soon after terrorists killed twelve people at the offices of a French satirical magazine called Charlie Hebdo.
The #Feguson tag started trending after a grand jury decided not to indict a white police officer who shot and killed a black teenager, Michael Brown. These events are a few years old and it would be interesting to see how many tweets were generated so far in 2018 by the #MeToo, and #NeverAgain and #MarchforOurLives.