The one which is available to you.
I started out with Delphi, which is nothing more than proprietary Object-Pascal. It comes with the VisualComponentLibrary and FireMonkey, which are great for WYSIWYG forms application development.
In my professional life, I used Electron at one point, which puts together NodeJS and Chromium so you can use HTML, CSS and JS to create an application. I used it to refresh some of my older C#.NET applications, which may have been partly written by students and needed rewriting anyway.
Lately, I am playing around with the Sciter library (again HTML, CSS), and Rust as the language powering it.
I know that you can use Qt and GTK from a range of languages, like C, C++, Python, etc., and they are used to build entire DEs and DE tooling. Also there is Flutter, which you could use from Java or Kotlin.
As you can see, you have a lot of freedom of choice. Use whatever language you know, choose a modern framework, and get developing!