I'm assuming you're looking for a CSS framework, and not something like a component system for React/JS.
I really dig Semantic UI and the way they define their classes. It makes it easy to create an entire themed site without altering/overriding too much of their CSS. And I think it looks more modern than Bootstrap. It doesn't have an official built-in calendar module, but the community has one.
But Bootstrap is pretty standard, everyone loves that one. You can also check out Bulma, which pimps itself as a modern framework.
I tried Kube and didn't like it. I didn't like their grid system, or the way you override CSS. Foundation is a bit oversimplified, it'd be a better base to modify. I didn't like Materialize or Material Web Components. Materialize was broken in half the browsers (dropdowns should always work), and the Web Components were missing half the components I needed (which Materialize had -- but executed poorly).
You can check out this list on Github of CSS frameworks to see some others I didn't mention (like Primer from Github).