Darkseid's New God physiology and Omega Beams give him the edge over Olympian divine power.
Both are supreme rulers of their respective divine/cosmic hierarchies. Zeus commands the Olympian gods; Darkseid rules Apokolips and wields the power of a New God.
In DC continuity, the New Gods and the Greek gods exist in different but comparable tiers. Darkseid's Omega Beams provide an offensive advantage that Zeus's lightning bolts can't quite match in terms of homing capability and existence-erasing power.
Zeus's divine magic is powerful, but Darkseid has faced and overcome magical opponents before. His intelligence and strategic patience also exceed Zeus's, who is often portrayed as impulsive.
Omega Beams, multiversal true form, superior intelligence and patience. New God physiology exists at a tier designed to challenge the entire Justice League.
Zeus is king of the Olympian gods with reality-altering thunderbolts. Divine magic is a potent force that Darkseid can't fully ignore. Zeus has defeated cosmic-tier Titans.
Darkseid takes a narrow win through Omega Beams and superior strategy. Zeus is a formidable divine opponent, but Darkseid's combination of cosmic power and genius-level planning gives him the edge.
Darkseid also fights
Zeus also fights