I really loved using Borland C++ Builder 4 (on Windows), around 2000-2010. I've haven't found anything so easy to use since.
https://www.concrete5.org/ was way ahead at least when I tried it years ago.
It allows for in-place editing and as a dev I could easily add more custom elements/blocks to be dropped and configured.
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Elementor for wordpress is decent regarding the responsive thing. But load times in the end suck...
I wrote a UI builder for OS/2 PM that targeted PM or Windows. It also had supporting libraries and code generators that let you pick many standard formatting and validation functions as well as hook in your own. That was around 1990.
Why are we still only specifying the layout in this day and age? We should be able to spec most of an application 'in the UI builder' and only add the data processing parts, even the network access or persistence should be wired for us.
Can you tell us a little bit more about what your building? Is it web or native?
XAML (WPF, Silverlight, UWP, WinUI) is the best I’ve used.
What kind of UI are you building? What platform are you targeting?
It spromotes atomic design system
NeXTSTEP Interface Builder. It found it's way into XCode, but the constraint system with autolayout just got tedious.
In 2000s I really liked Glade for Gtk+
Actual source code is always the best despite the inconvenience.
Quasar.dev
Lazarus and Delphi have the most intuitive UI builders of all the tools I have every tried. Lazarus components can be anchored relative to the parent and to each other. Latest Delphi has various layout panels - stack, relative, grid. Both tools feel fast and responsive (even on older hardware) which is something that I haven't experienced with any .NET or Java tool I had to use. Both tools have been around for a long time and haven't changed or abandoned underlying technology every few years so projects built 10 years ago can still be used with latest versions, sometimes just recompiling is enough.