Bootstrap alternatives, which one do you prefer and why?

Bulma is amazing and easy to use. Also 0% JavaScript

My personal favourite CSS library is Bootstrap. Apart from that, I have worked on a few more CSS frameworks.

Tachyons - link

• Very lightweight.
• Highly adaptable and customizable. You can definitely build your design system on top of Tachyons.
• A wide range of components. And a huge number of color combinations.
• Supports Grid system. Mobile friendly.
• It is super easy to build components with modern JS frameworks. Ex: React.
• CSS and layout debugging.

Semantic UI - link

• Huge documentation with lots of examples.
• Uses Flexbox.
• Wide range of components. Advanced framework with functional components.
• Adaptive to modern JS frameworks. Works amazing with React, Meteor, Angular and Ember.
• Preprocessor support - LESS.
• Built-in three themes - Default, GitHub and Material.
• Built-in icons set. Over 900 icons from FontAwesome.
• Responsive.

Pure - link

• Lightweight.
• Good documentation.
• Responsive.
• Predefined set of themes.
• Highly customizable.

All the above CSS frameworks are open source. And they are widely popular in the community. Other than these there are multiple frameworks available. But I have not used them that much. But here you go,

• Material Design Kit
• UI Kit
• BaseCSS
• Foundation
• Primer
• Skeleton

I found this link on GitHub, consisting of various frameworks. It's really good.

I hope my answer helps.