Old Kratos from the Norse God of War games takes the trophy for the strongest Kratos. The character is older, wiser, more emotionally intelligent, and has the memories and guilt of his past to tie him into being better.
There is no old kratos or new kratos or young Kratos. Just the same Kratos. He is as strong as Kratos has always been.
He's still the same god, but with more experience and wisdom. Kratos 1-2 was pretty damn naive on the fact that he lost so much because his anger got the better of him. This Kratos is by far the most powerful.
In many aspects, Atreus feels even stronger than Kratos, but that might have been done on purpose to make players feel more at ease in the Atreus sequences. So, while Kratos has been getting progressively weaker with each new entry, this is only part of the series' plot progression.
Another thing people bring up is that GoW3 Kratos is by far stronger but someone mentioned that Greek gods get stronger as they age, chronologically that would make GoW4 Kratos stronger. Others also mention that since Kratos has taken a semi passifistic way, that he isn't ever using his full potential in GoW4.
While it is true that the hammer would have been an incredible asset for the player, it cannot be wielded or even picked up by Kratos (nor Atreus) for several reasons.
By the time we see Kratos in the series reboot, the man who once fought against gods and monsters on a daily basis was more of a nomad than a warrior. Fatherhood and losing a second wife have also taken a toll on Kratos, limiting his once-legendary strength to a fraction of its potential.
In God of War Ragnarök, it's likely that Kratos is around 1,055 years old. He's something of an old man, but as a demigod, he is still more than young enough to hold his own and then some in battle.
Zeus, however, weakened Kratos and tricked him into abandoning his godly powers into the Blade of Olympus, which Zeus used to kill Kratos. Although he overcame all obstacles, Kratos was stunned at Zeus's betrayal and swore revenge as he died.
Poseidon would come close to beating Kratos as he made full use of his underwater powers by projecting himself as a giant monster. It makes sense that he would be so difficult to beat, considering he ruled the entire seas and had been around for longer than even Zeus himself.
1 The Dagda
The Dagda is another Irish god, the King of the Tuatha dé Danann, and a mighty deity whose power is unmatched.
One of the key rivals Kratos had in the God of War series is Alrik, who is the leader of the Barbarians. Alrik has a bone to pick with Kratos as the Spartan leader supposedly killed his father. Despite dying after battling with Kratos, Alrik escapes the underworld to give Kratos one of the hardest fights in his life.
Fimbulwinter lasts three years, so by the time Ragnarök takes place, which is when the game will be set, Atreus will be 14 years old. So if 14 is then added to the Ghost of Sparta's age, Kratos is ~1064 years old in God of War Ragnarök.
He is a Demi-God [He gave up his God Of War powers], so while he ages more slowly, he still ages. This. When he says he's a god, he more or less means, he's part god but it's easier to just say he's a god. He is older for sure.
Doom Slayer would easily get the upper hand early on thanks to his range. His weapons can unleash Hell on Kratos, thanks to the sheer variety of ranged pain at his disposal.
He's killed 19 Full blooded Gods.
I would say that would be Hephaestus (Vulcan) the smith god of Greek/Roman mythology. He is the only god in perhaps, any mythology, to be crippled.
In many aspects, Atreus feels even stronger than Kratos, but that might have been done on purpose to make players feel more at ease in the Atreus sequences. So, while Kratos has been getting progressively weaker with each new entry, this is only part of the series' plot progression.
Zeus possesses more power than Odin, yet Kratos defeated him. Lol Thor and especially Odin are much much older than Kratos.
Hercules is the older half-brother of Kratos, and shares many similarities with him. Both warriors mercilessly killed their wife and children under the influence of gods (Hera for Hercules; Ares for Kratos), having been forced to serve Olympus for a number of years in order to hopefully gain redemption.
A common consensus is that training to achieve Kratos's physique would likely require a smart approach of exercises such as overhead presses, rows, pullups, and deadlifting.
So then, in order to break fate, Kratos must make the choice not to kill any of the Norse deities in God of War Ragnarok and choose to trust Atreus to take care of himself. This is precisely what Kratos does and is the reason Kratos doesn't die in God of War Ragnarok.