Blasting away time and perpetuating an endless time loop than ensures Zusak can never be defeated, creating branching timelines where the MC tried different ways to thwart him.
I can't decide if I like the Hero ending the most of the Martyr ending, both are quite appealing, although the latter is a little more poetic.
But the former was definitely the most fleshed out since it comes with an epilogue regarding the further fates of secondary characters.
The Villain ending on the other hand... I suppose it has its fans and although the MC and Zusak can be seen as parallels to each other, the two never really developed a relationship outside of the MC wanting answers and Zusak regarding them as a thorn to his side.
One could argue that learning about Lachlan gave the MC some greater understanding of Zusak's motives and drove the parallel between the two ever further (both are driven to change Daria or even the world due to the death of family) but the MC's conscious choice not to overthrow Zusak and take his place as the Villain and instead decide to join him which Zusak seemingly accepts just seems... I don't know, the MC is joining the man that killed their parents and usurped their birthright.
Unlike in the Hero ending we don't get an epilogue detailing what happened to the secondary characters, it can be assumed they either perished or its possible the MC spared them, perhaps even setting up their infant child to one day grow up to slay both them and Zusak.