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.
In my view the weight is not so much a factor as long as it's not so heavy that you can't swing it effectively. If you're in a situation that you need to defend yourself with a melee weapon you pretty much only have one chance. If you don't drop it with the first swing you're wrestling next.
You make a good point though. A plain old hatchet will probably get the job done just aswell and trading some stopping power for accuracty might not be a bad trade. I don't think a sledge hammer for example would be any good because of its weight but atleast the Fiskars splitting axe I have is nowehere near as heavy and I can swing it around quite easily.