JavaFX by far. The API is probably the nicest I ever used of any toolkit, it's got a lot of features, is maintained by multiple companies, is mature, has a well integrated WebKit control, good multimedia support, can do 3D effects, uses CSS, has a UI designer, is stable, runs on embedded, mobile, desktop and can be "projected" to web browsers using JPro, and most releases are just bug fixes these days.

Currently fully FRP UI is quite popular (Jetpack Compose, ReactJS etc) but frankly whenever I've read such codebases I've been somewhat unimpressed. You can make JavaFX work that way with a bit of extra utility code, and there are places where that approach makes sense, but fundamentally a well engineered OOP UI toolkit is in its element. The functional reactive approach seems to create quite a few problems. Whether it creates as many as it solves, I reserve judgement as I haven't had a chance to write a large GUI app since they came out. Still, I've written a lot of them in the past and would definitely be tempted to stick with JavaFX despite having worked through the Jetpack Compose tutorials.

I think Jetbrains are a great example of Swing done right, who's doing the same with JavaFX?

Browse through the app gallery at https://www.jfx-central.com/home

(note: jfx-central is itself a JavaFX app, again running with JPro to be projected over the web, but you can also download it and run it locally).