What are the open design systems you prefer to use and why?

Design systems offer numerous benefits for individuals and organizations involved in the design and development of products or services including consistency, efficiency, scalability and more. What are some of the design systems that you prefer to use and why? We looked at a lot of amazing design systems but we thought they're not quite as beautiful and as easy to use, that's why we crafted our own carefully crafted design system called Sigma Foundation which is an open library and you can check it out theSigma.co