I thought it was pretty clear that it was the hammer to the head
Why? Because of sitcom amnesia logic? I have trouble lowering my expectations to that level. And what, did Superman never hit him in the head in their huge fight at the top of the season?
I also find it less than credible that the hammer blow "killed" Doomsday at all. While the hammer was built to be able to fight Superman and was thus comparable to his peak strength, it's a bit hard to credit that it could surpass it. Really, the whole thing just seemed contrived.