"Without weilding a magic sword, this foolish Greek warrior cannot face me!" Or something.
I'm not sure. I've actually not seen the truly high end feats that some people seem to have seen for Aku, because I never finished the series. Things got hectic during that part of my life.
Currently it seems to me Zeus should win, but that's possibly because I don't know enough about Aku.
It took 3 of the most powerful Gods from different pantheons to hurt Aku. Odin, Ra, and Vishnu. And even then, they only slowed down the entity he was before gaining true sentience and demon/godhood of his own. After that, it took those same 3 gods to forge a weapon that could harm him.
Zeus alone probably can't pull it off.
haermm
A note on Aku being able to split his form: Yes, he can do so, with the exception of being cut by Jack's sword, which usually causes the severed portion to dissolve/burn away. The three gods' weapons were capable of harming the proto-being that spawned Aku, so I don't see why Zeus's divine power couldn't do the job.
They were powerful enough to chase him through space and and whittle him down to nothing with various forms of energy blasts, energy arrows, and the like. Before he became Aku he was just a shroud of pure evil and death, but they sensed his presence almost right after he fully formed.
When they killed him, the tiny piece left over caused the dinosaurs to go extinct. And while he was still just evil and had no sentience, Aku poisoned a large majority of the world with a forest of spikes. And would have basically encompassed the planet if not for Jack's dad. And keep in mind he could have done it faster, but he was just a being of evil who affected the things around him and wasn't actively trying to do more as he had no sentience at the time.
After Jack's dad accidentally gave him sentience, he nearly destroyed his entire kingdom in a few minutes.
Aku's also got pretty much massive elemental control, laser eyes, invulnerability, and even a sonic scream.
He's got a whole lot more to throw at Zeus than you'd think. And the only things in the show to hurt him are the 3 Gods and the sword forged by them.
I'm not questioning his versatility, I'm making a claim that divine power is antethical to Aku's very being. Odin, Ra, and Vishnu were all the heads of their respective Pantheons. Zeus is as well. It stands to reason he should be capable of harming Aku. Knowing Aku, he probably would try taking on Zeus with his shapeshifting powers, but once he realizes that those lightning bolts hurt, he'll be more cagey in his power usage.