Learn it here: Bootstrap CSS
Tailwind CSS is a utility-first CSS framework that is different from other best CSS frameworks like Bulma, and Bootstrap where you get pre-designed components, which you can use as a base for further development.
Learn it here: Tailwind CSS
Bulma is another best CSS Framework based on flexbox. It is a 100% responsive open-source CSS library. Bulma utilizes a system known as tiles to build grids, which makes the page sleek and smooth.
Learn it here: Bulma CSS
Pure CSS is built on Normalize.css and developed by Yahoo. It comprises a set of small, responsive CSS modules that you can use in every web project.
Learn it here: Pure CSS
Skeleton is a super-light or boilerplate CSS framework designed for the development of responsive and mobile friendly websites. Skeleton includes all of the standard components for responsive web design, such as the grid, aside from its small size.
Learn it here: Skeleton CSS
Learn it here: Foundation CSS
Learn it here: Materialize CSS
Semantic UI is another good CSS framework. It contains pre-built semantic components that are super helpful in creating a beautiful and responsive layout using human-friendly HTML syntax.
Learn it here: Semantic UI
UIKit is regarded as one of the best CSS frameworks. It is a lightweight, open-source and modular front-end framework for developing fast and powerful web interfaces. UIKit framework is simple to learn and is used to build user interfaces for iOS and Android-based web.
Learn it here: UIKit
Tachyons is a lightweight, responsive CSS toolkit (utility first design) that claims to be ‘built for designing’. Going by the name ‘Tachyons’ means ‘a hypothetical particle that travels faster than light.
Learn it here: Tachyons
This are recommendations from my personal experience and everyone can have a slightly different one. Opinions are highly welcome.
Source: Biko Steven