it depends on what kind of widgets you really need, and which OS your app lives in. For traditional widgets, I like wnim on windows OS. If you need to support both linux and windows, you can have a look at nim-vcl and IUP ffi binding , both of them are based on matured GUI library.
As for web-based UI, I am often told which is a trend, but I never know how to communicate between nim and web UI framework with not so many codes and too many extra knowledge on JavaScript/HTML.
To be honested and not to be offensive, I will not use any new born libraries, or libraries based on third party library with a short history.
Just opinion from me, an amateur programmer.
It's not related to Nim, but about modern GUI development in general.
I remember good old days, launch Delphie, drag and drop some forms and buttons, write some handlers, click "Run". Done, you have your App with GUI.
Today... you have to do so many weird and unnecessary steps, configurations, asset, scripts, style compilation, frameworks and library configurations. The process looks like filling teeth through the asshole.