Ask HN: What front end framework/library is the best to learn as a BE dev?

A few days ago, I asked about Ruby in Rails and the general consensus was that it's great and boring framework that is a joy to work with, especially for bootstrapping.

My next question is more complicated because the FE ecosystem is expectedly even more crazy than in BE.

My goal is to invest some time in learning the basics of front-end so that I know enough to both build a traditional server rendered app and a modern SPA.

With so many choices, I have a feeling that React is by the very definition that boring technology and a pretty safe choice. In my case, employment doesn't matter though since I don't need to be hired as a FE developer so I can just learn the nicest one.

Svelte seems to be a pretty popular choice, but I have heard that the changes are still pretty major so not sure whether it makes sense to give it a serious try.

What library/framework would you choose and why?