jsFiddle - is better than codepen, for trying out quick snippets and 3d party libraries
CodeSandbox - very powerful options to get started with different frameworks like Vue/Preact
CodePen is very popular, but it has occasional glitches importing 3rd party libraries. More importantly, it is not great for sharing your code snippets, because you can not set the size and configuration of the screens, the JS, CSS, Console, etc - which ones you want expanded, which ones you want horizontally or vertically.