Round 4: Guts. And it’s not even close.
I agree the things Guts has done to stay alive or even to slay a demons are insane, especially in the first chapter where he slept with an apostle and kill it haha.
Zombie man is quite the crazy one too hiding a gun inside his ribs. And I love his axe/guns etc.
Round 1: So Guts is a much, much stronger fighter but as we saw with Pure Blood vs Zombie Man, you can be a much, much better fighter than ZM and still lose to him if you can’t get around his healing/immortality lol. So without a way to actually permanently kill him, Guts loses this in the long run.
Yeah even if guts cuts him in pieces Zombie will live. Only way is demon magic or putting bombs in Zombie body when he is down (and even that... Homeless destroyed Zombieman with beams and Zombie stayed alive like a boss.
But zombie is also a good fighter, with strength a little above human level (can bend steel bar etc) and his main assets are his guns with high caliber bullets they can do some damages to Guts.
I think that Zombieman will be quite effective in Berserk and can kill a good number of apostle.
Guts in OPM is likely a top class A level heroes.