The Incredible Hulk has long been considered the strongest there is, but - outside of Earth - there are still those who are considered even stronger. Powerful cosmic entities dominate Marvel's landscape and it stands to reason that plenty would make a meal out of the Avengers' not-so-jolly jade giant, given the opportunity.
Only, that would be the case, had Bruce Banner's alter-ego not demonstrated time and again that he's very much capable of punching above his weight. In fact, the Hulk is so powerful that he's been known to go toe-to-toe with other-wordly threats, and - on more than one occasion - come out on top.
These duels rarely ever feel disingenuous either. In fact, for the most part, the Hulk has earned the right to be called the strongest there is, as it's never a case that he defeats a given foe through some random contrivance. Instead, whether because of his surprising ingenuity or his own increasing strength, Bruce Banner's other half always finds a reasonable way of besting an enemy, to the point where it's unlikely readers will ever feel cheated by seeing the Hulk come out on top.
And sure, a lot of these victories can be chalked up to the contrasting whims of Marvel's many writers, but there's still a reason why the Hulk's accrued such a fearsome reputation...