Everyone has heard of the Hulk. In the Marvel universe, he is the testament used to define how strong or how durable something is. Example being his fight with Thanos in the Infinity War movie. His fight was used to prove that Thanos could stand up to even the mighty Hulk and overpower him.
The Endgame movie also introduced that there are different versions of the Hulk. We are given Professor Hulk in that movie. He is not as strong and doesn’t have the power-up due to anger effect. He still has great strength, moreso than even Thor. With the added benefit that he is still in control of his actions. Some interpretations of this Hulk sat while getting angry he will turn human. So the Hulk’s mind in the human body.
Those are just the two introduced in the recent movies. There are many more. The first time we see a controlled and calm Hulk was a grey version of the Hulk referred to as Mr. Fix It. As that was the name he took on as he hid out as a repair tech in Vegas. Still smart, and having the mind of an engineer, but nowhere near the level of intelligence of the Professor Hulk, nor the strength of a Savage Hulk.
For the record, for anyone who cares, Savage Hulk is the default Hulk. Bruce turns into him when he loses control and he gets stronger as he gets angrier. This version is the iconic version and is most often depicted. He, however, is not the strongest.
Maximum Hulk, or sometimes just Hulk, is a situational version of the monster. Under normal circumstances, the green beast has Bruce holding him back in some form. In this version, Bruce is removed or ‘killed’ inside of the creature. We see this in comics when the Phoenix does this before setting his rage to the max. In the Hulk vs Thor movie, Loki physically removes the man from the monster.
When this one is out to play he has no limits. He never tires and is the definition of the adage for this character.
“The madder he gets, the stronger he gets.”
Of course, during these times, he is no one’s ally. Just a weapon that you can hopefully point in a direction and hope to deal with the fallout. The time with Phoenix he is ‘sent away’ by the cosmic entity called “Onslaught”. A creation of the fusion of the minds and powers of Professor X and Magneto. The second time required Bruce to be willing to refuse the creature. Bigger feat it required Loki’s daughter Hel (big sister and ruler of the dead) to willingly give up a soul.
That is still not the most powerful version of the Hulk. World War Hulk is the deadliest he has ever been. Coming from the Hulk monster learning how to fight and a situation where both Bruce and Hulk were upset. Cliff notes are that they lost their wife and child. (Not Betty Ross, he was on another planet at the time.)
The result is that they came back to Earth to destroy a group called the Illuminati. A collection of the most powerful, influential, and intelligent Marvel characters. Ironman, Professor X, Black Panther, and Black Bolt. All of whom are considered dangerous, but none stood a chance against the rampage he brought.
The fights were destructive on a whole new level. On the moon, the Hulk stood up to Black Bolt. (He is king of the Inhumans and a whisper is strong enough to vaporize a building.) The king started the fight like he always did. Blasts from his voice, which is his inhuman power. The Hulk wasn’t fazed by the attacks. The king did something up till this point he had never done, he shouted. The Hulk continued his march towards his opponent.
All of the fights of World War Hulk were like this. It was only when he learned the truth of their deaths did he stop. The only person to fight him to a standstill was Ghost Rider. The spirit of Vengeance decided that Hulk was not guilty in this case and force Johnny Blaze to leave. The most humorous was Darwin (see First Class for cliff notes), tried to stop him. After a brief bought his powers decided the only way he could survive the Hulk was to teleport away.
The last two I want to mention is Maestro and Hulk of 90210 (Old Man Logan universe). Maestro is the Hulk from a future where the world is destroyed by nuclear fires. Instead of killing him as it did to most heroes, he absorbed it and became stronger. Our Hulk ended up fighting him in two very good stories. The end result being Maestro being killed by the very bomb that created the Hulk in the first place (closer to the epicenter).
Maestro was physically stronger than our version of the Hulk. How much is hard to determine as he was fighting Professor Hulk in both cases. Him fighting a Savage Hulk might have a different outcome. He was also more experienced and could think, unlike the more bestial Hulk. One of the stories has him ‘recruiting’ alternate reality versions of the Hulk so he could conquer the Multiverse. Showing he retained his intelligence.
Old Man Logan’s version of the Hulk is dark and evil. Going to start the explanation there. In a world where the villains teamed up and killed all other heroes, he became a villain. At first, shown in his human form, but retaining strength and durability, he is in charge of the Hulk gang. The gang is entirely made up of his inbred children. Which he created by raping his cousin She-Hulk. Told you, dark.
When he transformed he was much larger than previous versions. Able to swallow Wolverine whole. This also shows he may have lost some durability or healing factor as this also resulted in his death. Logan cut his way out from the inside.