User blog comment:B214/Volume 22 spoilers/@comment-95.90.227.198-20170201191840/@comment-3435913-20170202002608

I don't know how to measure up strength in the DxD universe, but I'll just explain the difference through mythology;

Indra is the King of the Hindu Mythos; he controls the weather as his primary attribute so he basically call the shots on whether the lands beneath him will prosper or fall to decay by calling upon the storms, rains, thunder, and even flooding rivers or let them dry out. He is powerful to the extent that he influences the mortal world and that he also rules the Hindu gods who reside in Saudharmakalpa (First Heaven).

Shiva is a Primordial God (as in he existed before most of the other gods) alongside Brahma and Vishnu (the Trimurti). He is the incarnation of Destruction and Rebirth, and is the more powerful of the Trimurti; Brahma creates, which neither implies whatever he makes has any strength or durability so to speak. Vishnu preserves, but likewise cannot enforce continued existence against nature. Shiva obliterates what he desires to be destroyed, but also has the added benefit to recreate or revive what he has destroyed.

To compare the gap between the two is difficult, but while Indra works by the laws of the universe with his powers (aka he can enforce a reaction by creating an action over the weather, thereby changing energy to suit his needs), Shiva's breaks the laws (rather than changing energy, he physically destroys it and can recreate it exactly the same).

On the gap between Ophis & Great Red and Shiva, it's entirely different;

Ophis is infinity, as in no matter what she (i'm using 'she' because obvious reasons) always is, always was, and always will be infinite. Infinity is the definition of never-ending, so Shiva cannot destroy something that will always exist eccentrically.

Great Red is the incarnation of dreams, and likewise cannot be destroyed also by Shiva. He is referred to as the "Illusion of Dreams" as in he both exists but also doesn't in a sense, so while he can be destroyed, he also cannot because he exists beyond the physical realm.

Ophis and Great Red are like an irresistible force paradox (an impenetrable shield and all-piercing spear) where both are almost impossible to destroy, but can only destroy the other that the cost of themselves, which is why Ophis created the Khaos Brigade in order to create an alliance of beings to give her the advantage in a potential fight.

Trihexa also follows the same logic, and forms a Three-way super power between then, where 2v1 at full power would be enough, but never 1v1v1.

Now comparing Issei and Vali is even more difficult because the two of them are similar but are also different;

Vali is a half-devil, but more importantly is a descendant of the 1st Maou Lucifer and Lilith (the mother of devil-kind) which is shown by his display of eight devil wings which emphasises his large capacity of natural demonic magic wielding and reserves. Also, but his human half he was able to bond the Longnius Sacred Gear, Divine Dividing, to his soul and was able surpass previous wielders in unlocking Albion's signature technique [Reduce].

Issei is a humanoid dragon recreated from Great Red's flesh and Ophis' power bounding his human soul into a new stronger body. He has unlocked Ddraig's signature move [Blaze], and has access to Albion's [Divide] and [Reflect] abilities also. He also has the dragon-slaying sword, Ascalon in his utility.

It's difficult to say who is stronger when you also compare that the two have access to [Diabolos Dragon], but we can get a clearer idea of the two in terms of power;

Vali is stronger because he is naturally accustomed to wielding power having managed to surpass many by his own strength.

Issei has done the same, but has gotten to the point where he cannot handle it as well having taken in Ophis' power of infinity and Great Red's power of dreams which became too much to handle when he first entered [D∞D] the first time, that he ended having to have a limiter placed in him so he wouldn't overdo it next time.

But, that doesn't mean that Issei cannot overcome Vali in the future, it's just at the moment he is being restrained from reaching his full potential compared to Vali who can use all his strength whenever he wishes. So right now, Vali is the stronger of the two by a lot, but Issei will eventually far surpass him in the future.