For rapidly prototyping an idea, I have yet to find anything that was as good as VB6. Drag a button, write code. Want to change things about the button? Use properties, that live update the GUI without recompiling. It was so simple that a reasonably intelligent person could grasp it in an afternoon, but in the hands of a capable developer could do some very impressive things.
It was also a fun game to hunt for OCX controls that you could use in things that were downloaded or came on random disks or CDs.
I really feel like VB6 was the peak of that development model and we've been moving away from it since. And I get some of the reasons why (just look at the mess that comes from trying to do anything with Xcode storyboards and version control.) But for just rapidly trying out an idea, I have yet to find anything anywhere that was as good as VB6 was.
You can try Lazarus[0], which is essentially an open source Delphi, itself being a more "serious" alternative to classic VB using a real native compiler and more robust language (Object Pascal).
It is more complicated than VB6, though nothing that can't be figured out by playing around with it and the included examples (and IMO it has by far the best visual form designer - placing controls in a freeform way and then "tying" them together with anchors with padding, etc so that they resize properly feels much more intuitive than using layout managers like in other toolkits that emphasize more on making stuff with code and have WYSIWYG editors as an afterthought at best).
Sadly there isn't any open source tool that i know that approaches classic VB's simplicity (though when it comes to that i am a bit more of a VB4 fan - i think by the time VB5 was introduced, the UI got a bit too complicated for the "form editor with a scripting language attached" that classic VB was at its core).
To be completely honest I haven't been involved in the Windows ecosystem in probably 15 years so I am not really sure. I switched over to using Macs in 2005 or so. I just Googled it and poked around and, yeah, at a high level it looks a lot like the old VB form builder.
The thing is, the form builder was just part of what made the VB experience so great. It was how everything tied together conceptually into a complete environment that, crucially, was very easy to understand and very approachable. The form builder was easy to grasp, language was very simple and straightforward, and the documentation was just astounding (to this day I have yet to find anyone who has as good documentation as Microsoft did in the 90s.) You could crank out a simple app in a matter of hours with it.
In the late 90s I got swept up in the dot-com boom and started doing web development. That's been my primary bread and butter since, although these days I have moved into a more management-focused role so most of the development I do is on my own time to keep my skills sharp. I have occasionally written things in Java (using Netbeans, whose form builder was very similar to VB) and Objective-C (again, Interface Builder reminded me a lot of VB).
I've been doing a lot of Swift recently for macOS development because it's such a fun language, but I haven't jumped over to using SwiftUI. It's just not ready for prime time yet, IMHO, especially for macOS apps. And I really just prefer building GUIs using a GUI builder like Storyboards or even old fashioned NIBs/XIBs, since most of the macOS apps I write I am the sole developer on, the version control issues with GUI definition files aren't a big issue for me.
Though I will say that I think SwiftUI has potential - especially with the live preview as you're building. It just doesn't work very well right now for complex macOS apps.
From a glance, it looks like it has the better features from either of HyperCard: message passing between "objects" vs. the class hierarchy, and VisualBasic: timers vs. the idle message, floating properties box vs. the dialog box.
<snark>It would be good if it has an option/property for resizeable windows "stacks" with some of the auto-resizing, alignment, and guides like Xcode - maybe it does, I get an error every time I attempt to login to LiveCode. "Critical mass" in user communities would be the killer feature, too bad the LiveCode App doesn't come by default with every new iPhone.</snark>
Xojo is pretty slick, but 1) LiveCode crushes it in the forms design department, and 2) No Android support. They have been promising it since just after John Quincy Adams was elected, though it appears to be in beta.
