I'm kind of an anime/manga n00b, but I notice people on forums always talk about like who is 'best girl' (or 'best boy' or 'best guy' Idk) in any particular anime/manga series.
Then I noticed
in this early episode of Kaguya-sama a character Miyuki says that Kaguya is, what I understood to be something like, 'saikou no onna'. Google translate suggests '最高の女', so I guess that's it. The subtitles however translate as 'ideal woman'.
in this early episode of The Quintessential Quintuplets the male protagonist Fuutarou calls the female protagonist Itsuki 'saikou' and then the subtitles translate as 'best' (as in 'You're the best.')
So it seems that when people say this term 'best girl', it is actually based on some common Japanese term '最高の女' ? Please cite sources. I guess my google-fu sucks (I just googled "最高の女" "best girl") or there aren't really any primarily English articles that talk about this. Or maybe it just isn't the case.