I'm a bit torn between a splitting axe and a mace/morning star of some sort.
An axe offers better penetration and is useful for other tasks aswell but perhaps posseses greater risk of getting stuck. A mace is not as versatile but is highly effecive when making direct contact with the skull and is unlikely to get stuck. A mace is also easier to improvise and customize for individual preferences.
when you can't carry any number of single-purpose tools and can't be guaranteed long-term shelter in which to house them, you need things that do many jobs. I think that a hatchet or a tomahawk fulfills the role of something that can pop a noggin with minimal training or effort from the end user, something that will be useful during the 23 hours and 48 minutes of an average day where the tasks you need to perform to survive don't actually involve popping noggins at all, and is relatively easy to maintain both wrt sharpness and the ability to replace the handle as-needed.