Hi Tamir,
Unfortunately, this feature is not currently available. But it’s a great idea.
Will you please add your feature request here: https://storepress.fider.io/
BTW, you can manually do it for each product from here.
And you can use the default import/export feature of WooCommerce to bulk update all products.
Thank You
How to choose variation to be defualt marked
Tamir Oren
How can i make that one of the variation will be default selected on all products