Game director Jeff Kaplan has released data regarding the top 10 most-played heroes for each skill tier as of the start of the current season and, based on these results, it looks like D.Va is currently the most popular hero in the game.
According to the data that Kaplan posted, D.Va is the most picked hero in five out of seven competitive tiers: Bronze, Silver, Gold, Master, and Grandmaster. In the two tiers where she is not at the top of the list, Platinum and Diamond, she comes in at a respectable second place, which still means players at those levels will see a lot of her regardless.
When it comes to designing a competitive game, pick rates are a fairly good metric for determining what heroes are balanced, strong, or perhaps weak during a certain patch. It can also help dictate what kind of metagame is most commonly seen in higher level play.
For quite some time now, "Overwatch" has been in what is called a dive composition meta. A dive composition is basically a lineup made out of aggressive and highly mobile heroes dive toward the enemy backline and take out their squishier heroes as quickly as they can. As one would imagine, D.Va is a very popular choice in this kind of style, and her pick rate certainly reflects that.
It should be noted that this information is for the current competitive season, which just started this month. Much of this data will still favor the tactics that grew popular toward the end of the last season and does not take into account any recent major changes.
Blizzard has also announced a new hero in the form of Brigitte. As a support-tank hybrid, she is capable of controlling singular targets while leading the frontlines. Blizzard has yet to release her to the live servers, but her inclusion to the lineup could easily shake up the current meta and popular picks.