I've found Ubuntu's default, and "vanilla gnome shell" to both be pretty cohesive and "modern".
And at the same time, I've never really felt like Windows or Mac actually end up with a more cohesive UI than the various linux desktop envs. For every Qt/GTK theming mismatch, I find a Windows mismatch between apps due to Windows being 12+ generations of design languages and toolkits built on top of each other. (e.g. the 3+ distinct "current" windows control panel looks (11, then 10, then 7, then XP as you keep digging into more and more obscure settings). And apps typically "freeze" at the UI design when they're born. e.g. XP apps still look XP, and so on.
And on Mac, you have the (relatively!) small number of apps actually artfully designed for macos. And then you have all the other ones - electron, java-based, cross-platform Qt apps (which naturally look like Qt apps... just like on KDE/gnome).
There's of course various quibbles over font render, that have existed since time immemorial. I don't think any one platform really wins hands-down here, though it's my understanding that mac typically does the best (as long as none of the non-mac-native apps manage to mess it up).
I really think people just have double-standards at this point, where their "home" platform's flaws are minor, and candidates to replace it must be flawless. (I'll also admit I'm the same, though NATURALLY I think I'm right - i figure if everything is electron and mismatched anyway, I might as well have a free-as-in-freedom operating system under it. Nobody is putting ads in my start menu or advertising xbox game pass to me in my notifications.
There are a huge number of examples here: https://www.reddit.com/r/unixporn/
I used to use Openbox and compile my own freetype with patches but these days want to spend my time on other things, so I'm just using macOS which has the best out of the box experience with the lowest TODO list when setting up a new computer.
It's hard for me to imagine anything uglier than the above, but beauty is in the eye of the beholder as they say.