The one, the only, the default one.
Majority of LibGDX devs started with this skin, as it's easy to find with Goog your search engine of choice. And, well, it works.
Supports most of the Scene2D widgets (including the newest tooltips), with notable exceptions being the image buttons. Features no additional icons, so if you want fancy controls for turning music on and off, you might want to modify it.
Preview taken from LibGDX skins:
Seems to be the same as the whole LibGDX framework. But then again - in commercial products, you might want to try out something fancier.