In fifth place we have Primal Kyogre. This Pokémon is a great attacker and does a good job at what it does. Lets look more indepth on how it does its job.
Primal Kyogre's base stats follow the trend mega evolutions do in which it has a 100 base stat increase, so it has a 770 base stat total. It has a slower speed, which is not the greatest, and a below average defense for a legendary, but it has a great special defense, supprisingly high attack, and phoenomal special attack. If you could not tell, this Pokémon is a slow and bulky special attacker.
Primal Kyogre has a good moveset. In my opinion, there is a little bit left to be desired, but it definately isn't a bad Pokémon.
Origin Pulse is Kyogre's signature move. It has a base power of 110 and it hits both opponents, at the cost of a horrifying 85% chance to hit. This may seem decent, but if you use this three times, about the amount you would probably do, you roll the 85% chance six times, means a 90% chance to miss one of those hits. Water Spout is a 150 base power move that deals less damage the less HP you have, so after one hit, Origin Pulse is almost always better, but Water Spout does have 100% accuracy, making up for Origin Pulse. Ice Beam is a great coverage move as it deals super effective damage to two of the three types that resists water. Finally, Calm Mind is a great set up move that raises a Pokémon's special attack and special defense by one stage, boosting Primal Kyogre's bulk and damage output.
Primal Kyogre is the primal form of Kyogre, which has the ability Drizzle. Drizzle activates the rain weather effect. When Kyogre becomes Primal Kyogre, it gets Primordial Sea, which is one of the three primal weathers, so it can't be overwritten by normal weathers and it can overwrite the other two primal weathers, at the cost of Primordial Sea being able to be overwritten by the other primal weathers.
While Primal Kyogre is great, it is missing four things that I think would make it so much better. The first thing I think would make Primal Kyogre better is a better speed stat, as Primal Kyogre is relatively slow. The second thing is that Primal Kyogre could get Nasty Plot as an alternative to Calm Mind as Nasty Plot trades the special defense buff for another special attack buff, which I think is better for most special attackers. The third thing is that Primal Kyogre could get Freeze-Dry as another alternative to Ice Beam as Freeze-Dry has the bonus effect of being super effective against water types, the one type water and ice types can't hit for super effective damage. Lastly, Primal Kyogre is forced to hold the blue orb, meaning it can't hold items that would benefit it a lot like choice items, life orbs, leftovers, or any other items that could help it. The reason this one is #5 is that it has those flaws I mentioned, which could let it make the the top three if it did have those, which have basically everything going for them. Why this is under #4 is that #4 has a better moveset and an ability that counters Primal Kyogre.
Even though I made it seem like this Pokémon is only decent, the fact that this is #5 says otherwise. It can still deal incredible damage with Water Spout and Origin Pulse in Rain. If this Pokémon was still in the game, it would probably be #1 today.