Which countries would benefit most from an American brain drain?

There is vanishingly little US "leftist media", and none of it is concerned with a brain drain from the USA. It's mostly outlets in the historically center-right media, including some of those who went out of their way to kowtow to Trump immediately before and/or shortly after the election, that have done so. Unsurprisingly, because "brain drain" is a concern of their corporate capitalist constituency, and not so much of the anticapitalist left.

(That said, it is a real and measurable thing, irrespective of what political faction finds it most concerning.)