Hulk has always called himself the strongest there is in Marvel Comics, and there are good reasons why he is the most powerful member of the Avengers.
Hulk has unlimited stamina when involved in a fight. When battling alongside most members of the team, his fellow Avengers have limits to their peak-level performance.
There are no limitations to how strong Hulk is. His power allows him to gain in strength the angrier he becomes.
There is a lot about Hulk's powers and abilities that even scientists can't understand. One thing that helps Hulk in his battles is that he is resistant to psychics.
Wolverine's claws among other things can pierce Hulk's skin, but much like the mutant warrior, Hulk heals almost instantly from any injury.
Hulk will never give up. He is a relentless powerhouse and that was no more evident than it was with Worldbreaker Hulk in World War Hulk.
The problem with most of the Avengers is that they overthink situations. Hulk is a rage-fuelled monster who just sets off after a villain and fights without abandon.
Hulk is not scared of anyone. There was a moment in Avengers: Infinity War where he seemed scared to come out after Thanos beat him.
There are some things that can hurt Hulk, but the only things that take him out are when someone uses a gamma attack on him.
Hulk has a rage that increases his strength to unlimited levels. This rage was caused by childhood trauma.
The biggest reason that Hulk is the strongest Avenger of them all is that he is immortal. The closest that anyone comes to Hulk is Thor.