Is Yoona currently the top-idol actress ?

yes

hopefully not, she's bad at acting.. her face saved her

Quote from vinslow4

The reason is none of them can compete with non-idol actresses, so the media/fandoms created a new category to make these 3 the top.

Ironically, even within that category, they aren't even the very top, so fandoms either ignored the seniors idol-actresses, or narrowed it further by limiting to only 2nd+ gen.

But subsequent generations did not overcome even them