Measured in revenue, it has to be Fortnite: Epic is apparently printing money to the tune of $100MM/month from it. And anecdotally, over the course of just a couple months it certainly seems like every single person under the age of 18 in the United States started playing it.

depends on what sphere you're looking at. the overwatch league is poised to take over in the esports scene while fortnite is gobbling up the casual playerbase.

note that this is north america focused, i have no clue about europe, south korea, or china.