Elixir's/Phoenix' LiveView (and clones/similar approaches). It's been discussed plenty on HN, so I don't think I need to elaborate too much. But I'll just say that as I'm using it in more and more new projects and adding it piecemeal to existing ones, I'm constantly amazed by how much complexity just disappears, and how pleasant it all is to work with.
I do feel that it'll take another year or two for all of it to 'mature', though.
For one, I'm not a big fan of how it still relies on 'old-fashioned' templating, but I've been looking into Surface [1] as a solution to that (it uses a more React-like component-based approach).
I also find that there's often confusion about best practices. About what goes where exactly, asking myself whether to keep state in the top-level LiveView is best, or perhaps too much based on the old client-side React/Redux paradigm and less necessary now that fetching data is a server-side-only DB call away (and using PubSub for any inter-LiveView communication).
But even with some of these 'issues', it's probably the most fun I've had building interactive web apps!
Can you name a few of Phoenix LiveView clones/similar approaches in other languages that are actively developed and worth looking into?
ps: The BEAM is what makes LiveView so great, do not expect the same in the other environments.