Q:

Magento2 : What is the default table used by Bestsellers in the admin?

In the database, I know that there are 3 aggregated bestseller tables 

sales_bestsellers_aggregated_daily,
  sales_bestsellers_aggregated_monthly,
  sales_bestsellers_aggregated_yearly 

but which one of them is the
default?
See attached screencast image:https://monosnap.com/file/5N7Q0kR95cDKvjpAeD29MDEAdfxmn6

A:

Dashboard bestseller usese Report/Bestseller resource model that can be found in:
vendor\magento\module-sales\Model\ResourceModel\Report\Bestsellers.php

And as you can see in constructor that it uses AGGREGATION_DAILY by default
$this->_init('sales_bestsellers_aggregated_' . self::AGGREGATION_DAILY, 'id');

From the look of this, I can say it uses sales_bestsellers_aggregated_daily table for this report

