My specs of code playground is just about, whether I can see the source code of the demo page as minimum as the required code or not. JSBin probably the best at this time, it’s just that I feel I have no control to resist the automatic revision, so I chose JSFiddle over JSBin.
Never use Dabblet (already using JSFiddle and JSBin).
Just my opinion as a user, I feel CodePen too much inserting code that’s not necessary for the demo/presentation purpose, so that people will find it difficult to learn the code placement or etc from the source code, it forces the user to always open the editor version to determine which parts that is actually required. Probably not a problem for the advanced level but beginners?
CodePen good for showcase but not for the playground, I think.