Shopify Polaris is one I consistently reference: https://polaris.shopify.com/ https://polaris.shopify.com/ It's super comprehensive overall, and I particularly

Shopify Polaris is one I consistently reference: https://polaris.shopify.com/ https://polaris.shopify.com/ It's super comprehensive overall, and I particularly like the documentation for the components. For example, even for a basic Checkbox (https://polaris.shopify.com/components/forms/checkbox https://polaris.shopify.com/components/forms/checkbox), they provide sample code, accessibility guidelines, dos and don'ts, and related components. They have clearly put thought into how the system should feel overall; design systems are not just about the CSS of the UI pieces. It's also cool how they display prop types; if it's a more complex type, then you can click on the prop for details, and click even deeper if needed.