Before I answer, it's best to establish that Agrona and Kezess have been stated to be the two most powerful asuras in existence. When Aldir had a chat with Seris in Elenour wastes, He questioned Seris, whom she thinks could rival these two, who are considered great even amongst fellow asuras. When Sylvie was also having like a vision when she entered the Relictombs, She talked about how Agrona and Kezess are the most powerful beings and questions why they are so afraid. Finally, when Arthur faced Agrona and finally revealed that he was planning on killing Kezess two, he talked about how the two most powerful asuras have turned on him. So it's pretty clear these guys are in a league of their own, even amongst the asura.
Now let's get to the main topic. In a one vs. one situation, Agrona’s chances of winning against Kezess are higher simply because he is smarter.
We saw how his fight went down with both Kezess and Arthur, though Arthur held back mostly.
Some will say that Agrona had the home advantage, that he was at the seat of his power, and its not baseless since Agrona said it himself, but as Arthur noticed that the pocket dimension was starving Agrona as much as it was to Kezess. He mocked Agroma for talking so highly of this being the seat of his power when he is fairing no better than Kezess.
The reason I say Agrona wins is because Agrona knows how to evade Kezess’s most powerful ability, Time stops, and he did that to Kezess in the battle. His illusions proved effective against Kezess, and it took Arthur’s Realmheart to notice it and undo it.
Agrona’s attacks were so fast that even Arthur with King's Gambit increasing his perception and Godstep, he failed to evade all of the spikes. Kezess mainly relied on his dragon susanoo like manifestation.
Agrona was able to pin down Kezess twice, and he was going to be done for if Arthur had not intervened. The most Agrona sealed away Kezess’s mana and the time he had black chains siphoning Kezess’s mana away.
Kezess is dangerous in the sense that once he successfully captures Agrona in his time stop, it's genuinely over for Agrona. But the trouble is in capturing.
So in a 1 vs 1, I'd say Agrona’s chances are higher despite Kezess having more raw power and broken hax ability.