Just mostly one because Kyogre is mostly likely based on Leviathan, I figured it would be interesting.

Sorry, but I don't see how this is anywhere near a close match. If you look up information on the Leviathan, you can see that it is mostly depicted as just another sea creature and is actually relatively harmless. Kinda like a massive whale. Whereas Kyogre, and especially Primal Kyogre, is an incredibly powerful monster capable on controlling the weather on a cataclysmic scale. Kyogre can end the word by simply being awake. The Leviathan cannot.

Geesh. Its about time we disagreed on one of these, excluding the one where neither of us knew but we just choose opposite sides cause.

If you ask me, while Kyogre is indeed incredibly powerful and can be noted as being able to bring the end of the world simply by being awake, there are factors that need to be taken into account for this. Kyogre would be able to bring upon the end of the world by drowning it. A world of nothing by endless ocean and weather conditions beyond imagination.

Taking that into mind, I would like to point out two points. The first of all which would be this is a battle against each other rather than a battle of who can create the most damage onto the world. The second of all would be that while Kyogre can bring the end of the world by simply being awake, Kyogre's method is by drowning it with horrible cataclysmic weather conditions. With that being said, the 'end of the world' would be less applicable to sea creatures and other salt-water creatures. It can be said that the weather conditions would be able to kill and bring end to a good deal of sea critters, though with information that I will mention later, I do not believe it would be applicable to Leviathan.

Also a note on both, neither are taken as having a harmful nature. Kyogre is relatively calm of a pokemon as long as it is not near Groudon and Leviathan is relatively calm as well.

Okay, Im going to spend more time laying out Leviathan's notable skill set because Im sure that Kyogre is much more well known. Upon reading the description of Leviathan from the Tanakh (Hebrew Bible) it is notable that Leviathan's strength comes more in its physical strength than anything else. The first thing we hear is the solid and strong scales that trail down the back within the following quotes. "His back has rows of shields tightly sealed together; Each is so close to the next that no air can pass between. They are joined fast to one another; they cling together and cannot be parted." "The folds of his flesh are tightly joined; they are firm and immovable. His chest is hard as rock, hard as a lower millstone." "The sword that reaches him has no effect, nor does the spear or the dart or the javelin." "A club seems to him but a piece of straw, he laughs at the rattling of the lance." "Nothing on earth is his equal—a creature without fear." The second thing that we hear are the more oddity of ability it holds which are the ability to release smoke from the nostrils, light from the eyes, and fire from the mouth. "His snorting throws out flashes of light; his eyes are like the rays of dawn. Firebrands stream from his mouth; sparks of fire shoot. His breath sets coals ablaze, and flames dart from his mouth." Dont ask me though what much of a use breathing fire would do in this battle, especially with Primial Kyogre. The third thing to take note of is the strength of the jaws and teeth that he uses so he can "treat [Iron] like straw and bronze like rotten wood." Also "Nothing on earth is his equal" which we may be able to put the absoulteness of it a side by saying Kyogre isnt exactly included. And then the last piece to point out which may or may not come into use is that while Leviathan is a beast that is relatively calm, it is not a creature that would be frightened or back away. I wont quote for this one because I would be reusing quotes again and Im also too lazy to fill this place with quotes.

Then since I know we all know Kyogre's strengths and abilities, Ill go over them again but just breeze over them. Kyogre has the ability over the element of water and can bring upon cataclysmic weather conditions that can end the world in a massive flood just by being awake. With Primial Kyogre, this power is enhanced greatly and fire based attacks are eliminated. Fun fact, Kyogre can fly...... XD I didnt know that XD

Overall, I think that Leviathan would win in this battle. While Kyogre is insanely powerful in many ways and win hands down if all we were doing were ranking the amount of damage they could do in general, I do not believe that if they were to be pared against eachother that Kyogre would win. This is mostly due to the intense weather conditions that might kill other sea creatures, it would be significantly less powerful on Leviathan who has such a heavy defense as well as being a sea beast used to the depths of the ocean. With that also being said, nothing on earth is its equal so whether it is to be taken into consideration or not, that means that weather alone can not take it down. Following with that, most of Kyogre's power derives from its ability to manipulate and cause problems with water which while it would serve to be a problem for Leviathan, I believe that Leviathan would be able to deal significantly more damage with being able to get close up with its teeth that takes iron as if it was straw.