Meta heroes are usually the recently released Heroes or recently buffed Heroes, with overpowered skills, or the even Heroes that are most commonly or rarely banned on Ranked Game. Most of these heroes were either buffed or some heroes were totally nerfed in a row without a buff. They are the ones who possess a relatively higher pick rate and shift the hero picks for every game you play.
As of Season 17 (August 2020):
NOTE: The slot of each role has a maximum of 6 to 8, not exceeding it. This list is based on global statistics, but may be subjective and may also differ in different ranks or regions. We can't include heroes who have been mostly banned in Draft Pick.