While any Hulk is a major challenge, none seemed more impossible to defeat than the Purple Hulk, who possessed all the powers of the Avengers, New Avengers, and Dark Avengers. But unlike the gamma bomb explosion that created the classic green Hulk, this villainous behemoth was the result of meticulous planning, and a recent issue of Immortal Hulk suggests he may have been defeated more by chance than the Avengers' planning and skill.
Introduced in 2012's Avengers #24, the Purple Hulk is Norman Osborn, aka Spider-Man villain the Green Goblin. After the Skrull attack known as the Secret Invasion destroyed trust in existing institutions, Norman Osborn managed to replace Nick Fury's S.H.I.E.L.D. organisation with his own fascistic H.A.M.M.E.R. Thankfully, the Siege of Asgard exposed Osborn for the maniacal despot he was. But while he was forced into hiding, Osborn kept much of the support behind his former regime. When he reappeared once more, it was with the backing of terrorist organizations A.I.M., Hydra, and the Hand, as well as disgraced S.H.I.E.L.D. agent Carolina Washington, who had used her privileged position to collect the genetic information of key Avengers.
Washington's work allowed Osborn to develop super-powered soldiers, creating an army of Spider-Hulks and Invisible Giant-Men, but the greatest abilities are kept for Osborn himself. Equipped with a version of the Super-Adaptoid technology by A.I.M.'s Dr. Monica Rappaccini, Osborn gains the ability to absorb and recreate the powers of any individual simply through touch. Having absorbed the powers of his own Dark Avengers and the Vision, Osborn is recklessly attacked by the Red Hulk, allowing him to transform into the Purple Hulk - a foe with vast musculature, the ability to control his own density, and the power to replicate any super-human foolish enough to strike him.
At this point, the Purple Hulk was already so powerful that the Avengers saw only one course of action: overloading Osborn's powers by gifting him too many different abilities to host at once. Though the genetically volatile Spider-Woman and Kree alien Protector both hoped to overload the Purple Hulk, it was only the efforts of the New Avengers that finally caused his body to crumble. What's interesting is that Osborn's final appearance resembled the Fantastic Four's Thing, who had just attacked him along with the Hulk's son, Skaar. With recent issues of The Immortal Hulk reminding fans that the cosmic rays that created the Fantastic Four are basically Kryptonite for gamma mutates like Skaar, it's possible this - rather than a more general overload - was responsible for causing the Purple Hulk's physical form to collapse, robbing Osborn of his new powers and leaving him in a coma.
What's most worrying about this possibility is that it would mean the Avengers don't actually understand how they were able to take down the Purple Hulk, and may be vulnerable to the same Super-Adaptoid technology in the future. While it's possible Osborn was taken down purely by the varied mix of powers - a risk explained to him as he was prepped for the upgrade - it was only after Skaar and Thing's combined assault that he showed signs of instability. Given that cosmic rays recently destroyed the Hulk himself, it's possible this internal gamma/cosmic radiation imbalance, rather than purely the variety of powers the Avengers gifted Osborn, is what took him down. Recreating this attack in future without causing gamma/cosmic radiation conflict could simply create what Osborn hoped - a villain with all the Avengers' powers at their fingertips.
Thankfully, Marvel's evil organizations haven't worked together as successfully since, and Osborn's sins were recently expunged by the supernatural efforts of the Sin Eater. A one-time foe who seemed ready to demolish Earth's Mightiest Heroes for good, Norman Osborn's Purple Hulk was only defeated by passing on the assembled might of the Avengers, but if the villain's defeat was actually down to the specific heroes present rather than simply being overwhelmed, it's a gamble that may not work again should some version of the villain return in future.