Depends on your end goal. But I'd say Tk/Tcl and Qt have done alright for themselves. They'll never be confused for native, but for a solid GUI app, they're quite solid.

Then there's always Unity ...

I'm a Qt programmer. It's very very good because it attempts to look as much as possible like a native app rather than forcing a "cross-platform look and feel" like Google has been doing lately.

But it still feels wrong on the Mac. Little UI details which just "aren't done" on the Mac. Plus Qt is always a year or two behind on everything means that when MacOS alters or refines a UI element that Qt apps will still be doing things the old way.

Tcl/Tk. Yeah, I worked with it for several years. It was great to work with and highly underrated.

Safe to say I would never write a commercial consumer facing app with that.

libui looks amazing but it's not quite ready yet https://github.com/andlabs/libui

Definitely Unity.

Opengl

That’s pretty low level and I don’t think most people would consider that a toolkit.

wxWidgets