From my windows 11 testing I liked B00merang-Project/Windows-10 most.
I guess it looks most native, what I personally just hate is how terribly white it is and how you cannot tell selected tab from unselected tab apart because the colors are so similar (this is sadly "native" to Windows though). Also the tabs occupy too much space to my taste. Anyway, I'm not a Windows user so it's up to Windows users to decide.
@techee you can change to the toolbar icon size from within geany right?
Yeah, sure, I was just thinking it would be a good default.