Wolverine, the best-known member of Marvel's X-Men, has a legendary healing factor. The Canadian mutant has survived everything thrown at him from the wrong end of a steamroller to fights with the Hulk to full-on nuclear blasts. Wolverine is also effectively immortal, surviving to the end Marvel's entire universe. But even with all of these feats, the Wolverine of Marvel's prime continuity is beat out by his Ultimate Universe self, whose healing factor is somehow even more overpowered.
Marvel's Ultimate Universe was created in 2000 as a way to make iconic Marvel heroes more accessible to new readers. Ultimate comics created new, modernized versions of heroes like Spider-Man and the Avengers that were free of the 40 years of pre-existing continuity that defined their mainline properties. This meant a more streamlined, unified continuity that paved the way for the Marvel Cinematic Universe in later years. The Ultimate X-Men stripped away supernatural and cosmic elements, focusing on young people with superpowers who were hated and feared by the rest of the world, and that were as likely to dramatically clash with each other as with their villains. The details of Ultimate Wolverine's past were altered, but he was fundamentally the same character, albeit less loyal and more ruthless as a result of the edgier tone of Ultimate Marvel.
Characters in the Ultimate line tended to have powers that were subtly different from their main-line counterparts. For example, Ultimate Reed Richards doesn't have a human body plan; he has no internal organs, other than his brain, which he can stretch to increase his intelligence. Wolverine's powers were seemingly the same as his main-continuity counterpart until a revelation in the miniseries Ultimate Wolverine vs. Hulk that showed their limits are beyond what other Wolverines are capable of.
In the issue Ultimate Wolverine vs. Hulk #5, Logan wakes up in a SHIELD interrogation room where Nick Fury wants to have a face-to-face with him about his battle with the Hulk in previous issues. In order to keep Logan from lashing out, Fury takes "face-to-face" to an extremely literal degree, decapitating the X-Man and putting his head on a table. Fury took it on faith that Wolverine's healing factor would keep him alive with his head removed, but Logan's physiology surprises him: Wolverine not only stays alive, but also maintains homeostasis when his head is taken off his body.
Nick Fury speculates that Wolverine's "healing factor" isn't just a healing factor at all, but a survival mechanism that can change his body in response to his environment. This puts him beyond Wolverine's normal abilities into the realm of mutants like Darwin, whose body can also instinctively respond to necessary stimuli. Where could this lead? Unfortunately, Logan didn't get much of a chance to test out the limits of his strange powers, because Magneto annihilated every last cell of his body in the Ultimatum event, and then the entire Ultimate Universe was destroyed. But we might still see this even crazier form of the healing factor yet.
The main Marvel continuity has taken in Ultimate Wolverine's son, who shares his powers. And the Ultimate Universe itself has been teased to return. Maybe the next step for Wolverine's healing factor is to recreate his universe to keep him alive.