The saying goes, "Hulk is the strongest there is." Which Hulk, though? There are almost as many Hulks as there are Spider-people and there have been multiple incarnations of the main Hulk himself. They are not all equal.
Some iterations of The Hulk are mindless rage bombs while others are pacifists and don't even know how to throw a punch. To answer the question of which Hulk is the strongest, let's take two of the most powerful incarnations of the big green smashing machine (only one of them is green, however) and compare notes.
10 World Breaker: Oxygen Not Required
On the ride over from Sakaar, Hulk stays on his ship's hull to smash comets (not sure why they don't just pilot around them) and to beat up space pirates. The exact distance between Earth and Sakaar is unknown and the speed Hulk's ship can travel is never mentioned, so some guesswork is required. The safe money is on the journey taking at least a day.
For Hulk to hold his breath that long while fighting space pirates and smashing comets? Strength is not the right word but it's the closest we have. It's worth noting that he also fails to implode.
9 Blue Hulk: Flight
Imbued with the Uni-Power and upgraded to cosmic being, Blue Hulk has many terrifying additional abilities beyond just smashing. One of the most interesting additions is the ability to fly. Picture that for a moment. A flying Hulk.
Without doubt it would be an unusual sight to behold, but also scarier than a Sharknado, or roving gangs of oxygen-breathing sharks with legs and good cardio. Hulk being able to fly increases his potential kinetic energy and makes him stronger than a puny World Breaking Hulk.
8 World Breaker: Black Bolt's Beatdown
Black Bolt has wiped out Celestials with the power of his voice. He has torn space-time and even reality itself with it. When Hulk turned up on the Moon looking to fight Black Bolt, nobody expected him to get far. Imagine the horror of anybody watching then, as Hulk withstood Black Bolt's power and proceeded to punch him into a bloody pulp.
It's a feat a few levels above incredible and it makes Blue Hulk's whole flying schtick look relatively unimpressive.
7 Blue Hulk: Telekenisis
So it's established that Blue Hulk is cosmically endowed. Flying, rampaging Hulk is as scary as it gets though, right? Wrong. Dead wrong. Blue Hulk has psychic power and telekinesis. This power doubles his smashing capacity and makes him the strongest there is. No longer limited by having only two fists, Blue Hulk can use his mind as a legion of fists.
Not only are Blue Hulk's enemies worried about bones being broken, they also need to worry about their minds being smashed. There's the added concern of Hulk being constantly bombarded by the internal dialogue of anybody close enough to him. For someone with no chill, this would be a constant trigger.
6 World Breaker: His Warbound
One branch can be broken easily. A handful of branches is harder to break. Together, they make each other stronger. World Breaker Hulk has something that Blue Hulk does not, something that makes him stronger than cosmic power ever could. He has friends.
Or Warbound to be specific. It's kind of like Friends+ and deeper than just hanging out and watching movies. This assembled rabble of ex-gladiator aliens have Hulks back no matter what. Hulk can invade a planet and subjugate the natives; the Warbound will cheer him on, no real questions asked.
5 Blue Hulk: Amplified Strength
Blue Hulk's hidden Ace in this contest is cosmically amplified strength. It makes the debate over who is strongest a no-brainer. Even without the flight and telekinesis, simply having amplified strength makes Blue Hulk the strongest Hulk there is. Two Hulks, two different colors, one possessing amplified strength from a cosmic entity. Ladies, gentlemen, and friends beyond the binary, Blue Hulk for the win, all day. Then again, as we've already discovered, there are different definitions of strength.
4 World Breaker: Dr. Strange Loses Twice
Defeating Earth's Sorcerer Supreme brings enough bragging rights to retire on. Some of the most powerful beings in and beyond the Universe have tried and failed to get the better of Dr. Strange. World Breaker Hulk alone slapped him around. Twice!
After his first defeat at the hands of the Green Goliath, Dr. Strange bonded with the extra-dimensional monster, Zom. This is a huge deal because in the past, defeating Zom would require the Living Tribunal itself. Multiply Zom's powers with The Sorcerer Supreme's and it's unclear as to how anybody could ever stand against him. That is, until World Breaker Hulk snaps Strange's wrists and ends the threat.
3 Blue Hulk: Energy Absorption
While bonded to the Uni-Power, Hulk can absorb energy and gain extra strength. This is in addition to the already augmented strength the Uni-Power brings to those it bonds with.
It could be argued that there's no upper limit on how strong Blue Hulk could become if he were bombarded with enough energy. Not a cool thought when you remember he's prone to wild rampages through major cities. It's hard to imagine anything that could compete with an unlimited force multiplier like this.
2 World Breaker: Grief & Hate
World Breaker Hulk has a force multiplier of his own: his grief over the death of his wife and his hatred towards those he holds responsible. The angrier he gets, the stronger he gets. It's kind of gross to look at it this way but his dead wife makes him stronger.
Blinded by revenge and propelled by hatred, World Breaker Hulk is probably the strongest Hulk there has ever been. His grief, rage, and hatred feed off one another in an endless loop.
1 Blue Hulk: Double Team
Blue Hulk has a hidden strength beyond his nightmare-inducing cosmic upgrades. He has the voice of Bruce Banner in his head. For all of Blue Hulks' physical and psychic strength, he still has a tiny pea brain and constant burning anger. Those are two fairly big weaknesses, without the rational (super genius) mind of Bruce Banner to reign in the anger and to guide the pea brain.
The mixture of Hulk, the Uni-Power, and the mind of Bruce Banner in one body makes Blue Hulk the strongest Hulk there is by a long shot.