The Incredible Hulk is most known for two things: his immense power and his constant war with various personalities. Bruce Banner's mental instability has given rise to a number of gamma powered personas, most notably the Savage Hulk. For a time in the '90s, many of the disparate Hulk personalities were merged into The Professor, arguably the most powerful and resourceful version of the character, a true amalgam, with the strength of the Savage Hulk, the smarts of Bruce Banner and the cunning of Joe Fixit. Until the Green Skar personality came through thanks to World War Hulk, the Professor was regarded by many fans to be the ultimate version of the Hulk, and clearly his most powerful.
Al Ewing's Immortal Hulk, however, is bringing back the character's litany of forms, resulting in a new Hulk that rivals the power of even The Professor. Along with that physical power and a sot of Machiavellian demeanor comes the acquisition of Shadow Base, a resource that mirrors Hulk's allies in the Pantheon during the '90s. This may lead to both the strongest, and most resourceful version of The Hulk yet.
Professor Hulk and The Pantheon
The Merged, or "Professor," Hulk was the combination of the Green Savage Hulk's strength, the Grey Joe Fixit Hulk's thuggish cunning and Bruce Banner's intelligence. The result of Doc Samson's hypnosis, this version of the Hulk essentially had all of the character's previous strengths with none of his weaknesses, making him far more versatile than the Hulk had previously been. Thus, he was prone to even using weapons to accommodate his already outlandish strength.
Along with his own power, The Professor Hulk had allies that could more than keep up with him in the form of The Pantheon. The fittingly named group were the descendants of an immortal half-Asgardian named Agamemnon, who formed the team to respond to natural disasters around the world. Together, they face several dangers such as abducting alien princes, and classic Hulk enemies the U-Foes.
The Pantheon, due to their godly bloodline, were all notoriously hard to kill and had other, individual superhuman abilities, usually manifested in energy weapons. This made them useful allies for the already stronger than ever Professor Hulk. Strongest among them was Ajax, who was forced to wear an armored exoskeleton to support his massive frame. This came, however, with immense strength that both rivaled the Hulk's and increased with anger like his, too.
Immortal Hulk and Shadow Base
The Immortal Hulk persona that Bruce Banner has taken after seemingly returning from the dead is similar to the Professor in many ways. Most notably, he has a much higher intellect than the traditional Savage Hulk. He also at times incorporates elements of other Hulk personalities, less of a merger between them and more of a court over which he holds ultimate jurisdiction.
This Hulk, however, is most certainly not the benevolent Professor. Instead, this fusion of Hulks is controlled by the so-called Devil Hulk, once believed to be an inarguably evil persona. The result of Banner's deep desire for a father figure, Devil Hulk's demonic actions and appearance were the result of Banner being unable to process the idea of love without immense pain.
Devil Hulk claims that humanity is on the brink of destruction, and that he may just as well help teeter it over the edge. At the very least, he aims to avenge himself against all those who had wronged and hunted down Banner. Chief among these would be Shadow Base, the next generation of Thunderbolt Ross' anti-Hulk army, Gamma Base. Leading Shadow Base is General Fortean, who has an even deeper vendetta against Hulk than Ross did. After Fortean's untimely demise, however, Hulk assumes command over the military base, hoping to use it to further whatever plans the Devil Hulk has for the world.
The Green Scar Caveat
One final note before we compare the Devil and the Professor: we would be remiss in not mentioning The Green Scar, a personality that resulted after Hulk was sent off world, conquered another alien one and then was seemingly betrayed by his former friends. Thanks to the (supposed) death of the love of his life, the Hulk became a vengeful king bent on destroying his former home. As such, a very strong argument could be made that this was truly the most powerful Hulk of all time, at least physically.
He also had his own super-powered strike force, as well as an army of robots. However, Green Scar was the direct result of a very specific series of incidents. As a focus response, he was driven by vengeance, wrapped up in a very specific goal. Devil Hulk has a similar remit, but it has been more than implied that his goal is to change the world, not break it -- that implies the beginning of a rule, not a strike.
Also, the Devil (quite literally) spreads his machinations from the shadows (or Shadow Base, as it were), meaning he is playing a much longer, more insidious game than the bluster-fueled frontal assault of the Green Scar. As such, the Immortal Hulk's plan could prove to be much more powerful in its outcome.
Which Hulk is the Strongest There Is?
The Green Scar's inclusion notwithstanding, the combination of various Hulk personalities is an obvious parallel between Professor Hulk and Devil Hulk. The former, however, may still be the weaker form, due to his occasional transforming back into Banner against his own will (a development that occurred later in the run). Devil/Immortal Hulk, however, seems to be able to toggle between the strengths of each personality at will, using them in various situations like a gamma-powered Swiss army knife. Immortal Hulk is also just that -- immortal. No mater if even Bruce Banner himself dies, Immortal Hulk will revive during the night time, healing the body to its normal status.
Hulk's new allies may also be even greater than The Pantheon. He is joined by Doc Samson, Rick Jones and Betty Ross, all of whom are gamma powered themselves. Given his connection to The One Below All and seeming control over gamma radiation, the Immortal Hulk may be able to extend that power to his allies. The Pantheon, with the exception of Agamemnon, were not immortal, however -- though it was revealed they became mindless zombies upon death, they were later defeated.
There are also the various resources of Shadow Base, which give Hulk the scope to enact his plans on a far greater scale than before. While The Pantheon had The Mount, this time, Hulk is literally using an army meant to destroy him for his own gain. It's unknown what exactly Devil Hulk will do his new army, but as powerful as he is alone, he can now do pretty much as he pleases.