If it were a choice between the two Pokémon themselves, I would choose Kyogre. Water has always been one of my favorite types and I have rather fond memories of Kyogre.
However, if we were talking about their primal forms, I think Primal Groudon takes the cake. His ability completely blocks out Water-type attacks which is a huge benefit for himself and even more so if used in a mono-Fire double-battle team.
Primal Kyogre, on the other hand, completely blocks out Fire-type attacks, which isn't nearly as exciting. I would suggest that he'd work good on a mono-Grass team, but Grass teams usually function better in the sunlight - maybe a mono-Grass team that doesn't operate with weather, but I don't see that too often. I don't think he provides much benefit for a mono-Water team, to be honest.