Warning: contains spoilers for The Swamp Thing #8!
While Marvel's mutant hero Wolverine and the loud-mouthed mercenary Deadpool have incredible healing factors, both have limits, especially in comparison to DC's most impressive regenerative abilities - those belonging to Swamp Thing, the avatar of the Green. This is because, while each can survive incredible damage, and may be immune to dying from old age, their powers are ultimately only biological, while Swamp Thing's healing is far more complex, as explored in the hero's most recent series.
Recently, Alec Holland - the former Swamp Thing - sacrificed himself against the Upside-Down Man, saving Earth and his teammates on Justice League Black. While Alec is now more powerful than ever, bonding with the dimension known as New Myrra, he's also constantly working to prevent the denizens of the Other Place from breaking through into other realities. Consequently, his role has passed on to Levi Kamei, who has been exploring his new powers and role in Mike Perkins and Ram V's The Swamp Thing.
In the latest issues, Levi has been hunted by a specially assembled Suicide Squad, with members hand-picked by Amanda Waller to kill him, including Flash villain Heat Wave and the walking mass of hazardous chemicals known as Chemo. But despite facing forces that should have an immense advantage over his plant-based form, Swamp Thing has not only been able to heal from their attacks, but grow more and more powerful as he embraces the fact that he's not just an individual, but an idea. In The Swamp Thing #7, Levi is bathed in toxic chemicals by Chemo, but realizes that his access to the Green (the elemental force made up of all the DC Universe's plant life) can give him the tools to survive and immediately regenerate, even growing in size. But while this gives Swamp Thing incredible healing powers, it's only the tip of the iceberg.
As Levi was told in The Swamp Thing #1 by the vicious Pale Wanderer, he hasn't become a plant monster, but rather an idea. Like Alec Holland before him, he has telepathic control over plant life, able to grow and animate it into a new body (or even several) at a moment's notice. In the past, Swamp Thing has been able to manifest a body from plant life as meager as the rolling tobacco in John Constantine's cigarettes, and Future State showed it's even possible for the hero to manifest different elements of his psyche - such as his hope and self doubt - as separate beings. As Alan Moore wrote, Swamp Thing is "a ghost dressed in weeds" - a disembodied consciousness able to grow a body around itself as a matter of will.
That's not to say that Swamp Thing's body isn't real. An avatar of the Green is able to take action thanks to a balance between human and plant consciousness - if they think too much like a mammal, they lose access to their more fantastic powers, while if they think too much like a tree, they lose the will to act (the eventual fate of many past avatars, who form the Parliament of Trees.) When these viewpoints are perfectly balanced, avatars possess truly mighty forms that they can grow and enhance at will. This is Levi's realization in the most recent issues of The Swamp Thing - that the stronger his tether to the Green, the more quickly he'll recover from otherwise devastating harm.
While Wolverine and Deadpool possess powerful healing factors, they regenerate from whatever's left after they're hurt. In the recent House of X #4, Logan is killed via incineration, while Deadpool: The End reveals Wade's daughter has designed a weapon that will totally destroy his body, preventing his regeneration. These are extreme conditions, but they still don't compare to what an avatar of the Green can survive, since their 'body' is the entirely of the plant life on Earth (and, at full power, even beyond.) Wolverine and Deadpool may be able to survive a lot, but Swamp Thing's ability to process and regenerate from damage is unparalleled, backed up not by superpowered bone marrow, but the metaphysical support of the natural world.