Invulnerability or regeneration powerful enough to allow one to survive a nuclear attack of any real magnitude is fairly uncommon in the Marvel Universe, but the biggest of the big guns of the Marvel Universe can survive, if barely, a standard ten megaton nuke; though probably not under optimal conditions.
NOTE: A nuclear device utilized under optimal conditions will likely kill almost everyone on this list. When confronted by nukes in most stories, they are usually smaller, not configured properly, too far away or some other aspect which allows the reader and the character enough of a possibility of survival to allow suspension of belief. (See: Indiana Jones and the Refrigerator of Nuclear Resistance)
This is most assuredly an incomplete list: Through sheer toughness (armored skin, invulnerability, super-fast regeneration, or some combination):
The Hulk (the Abomination and likely the Red Hulk):
His sheer toughness and regenerative ability has allowed him to survive nuclear blasts over his entire history.
He is one of the few mortals in the Marvel Universe capable of surviving a nuclear blast without aid of technology of any kind. (This would probably include his son, Skar who has all of his physical characteristics and many of his mother's as well.)
One of the most powerful of the Superman-analog in the Marvel Universe, his powers have almost no equal on Marvel Earth-616, he has defeated the entire team of Avengers without breaking a sweat and has the invulnerability to survive a nuclear attack.
Blue Marvel is likely one of the most powerful mortals on Marvel Earth-616.
Technically, mortal, but extremely long-lived thanks to the Golden Apples of Idunn. His superhuman strength, a testament to his mother, Gaia, and his Asgardian physiology from his father, gives him a level of invulnerability far beyond most Asgardians.
With the energy manipulating powers of Mjolnir, Thor could survive common nuclear attacks with difficulty. He is likely tough enough to survive one even without Mjolnir energy manipulating powers, but just barely. I would recommend he get under cover like everyone else.
- His powers, barely understood, make him one of the mightiest mortals on Marvel Earth-616. He can survive nuclear attacks because he believes he can. It's that simple.
- When he is in his right mind, he draws from a well of nearly inexhaustible energy, making him as powerful as he needs to be.
The Juggernaut: Cain Marko
It is theorized that Cain Marko was, for all intents and purposes, indestructible as long as he was the Avatar of Cytorrak.
In no other ways superhuman, the powers he gained while using the Gem of Cytorrak made him able to fight and win against nearly any other superhuman on Marvel Earth.
His level of invulnerability was tested in struggles against the X-men and Avengers on numerous occasions.
The Juggernaut: Colossus
With the superhuman durability of his osmium skin (which might not make him quite tough enough to survive a nuke at close range) coupled with the power gained by being an Avatar of Cytorrak, Juggernaut Colossus is arguably one of the most indestructible beings on the Marvel Earth-616.
Only beings of comparable power or possessing high-order energy manipulation or magic are even able to slow him down, let alone stop him. The energy of a nuclear weapon likely pales in comparison to his avatar-enhanced form.
A cosmic hero (and another Superman-analog), but mortal as far as we know, Gladiator is imbued with incredible power. Easily able to best most of the Marvel Earth powerhouses, he is a one-man army.
His only weakness is his need to maintain his concentration to utilize is incredible abilities. Awake and aware of the threat, Gladiator is an easy candidate for survival.
Yet another Superman-analog, he is a being whose durability would allow him to survive, just barely a nuclear strike.
He would also be immune to the radiation and atomic particulate fallout. He would not enjoy the experience.
Utilizing some combination of their superhuman abilities, and/or coupled with their sheer intellect, these characters may have a chance of surviving a nuclear attack.
Magneto: Master of Magnetism
Utilizing his mastery of the fundamental forces of magnetism, Magneto has managed to create force barriers sufficiently strong to blunt the effects of nuclear attack on at least one occasion.
He would likely utilize surrounding metallic materials to increase his shielding against the heat blast of the initial explosion.
This would likely tax him to the limits of his ability but he would more than likely survive, battered but alive. He would need to be utilizing his power to repel both radiation and particulate matter.
With his powers of personal molecular control, he can alter his durability and resistance to injury enough to survive a nuclear strike.
He would probably not enjoy it and would likely utilize mass from the environment to ablate the damage from affecting him, the same way he can utilize that mass to change his size.
Given his level of molecular control, neither radiation nor fallout would mean anything to him.
Jean Grey: As Dark Phoenix
In her Dark Phoenix form, Grey flew through a star causing the star to supernova. She was unaffected.
Her other incarnations of the Phoenix were significantly less powerful, but the Phoenix force was capable of battling Galactus to a stand-still, so they may also be powerful enough to survive a nuclear attack, if just barely.
I would grant her the ability to survive only if she were with Mr. Fantastic and he were utilizing his fantastic genius to augment her abilities in some fashion.
Her force fields while amazing and powerful have a feedback component which means depending on the force attacking them, she feels a stress or strain while defending against that force.
I don't see her being able to withstand a nuclear attack without technological assistance, getting under cover, or some other means of blunting the raw force of the nuke. She has never shown such capacity even when she was free of psychological restraints in her Malice identity.
Of the Avengers lineups over the last 50 years, the following could make an effective attempt at surviving a nuclear blast.
Hercules: functionally immortal, super tough, damage resistant
Ms. Marvel: massive energy manipulation, but not likely powerful enough to survive a nuke at close range.
The Vision: assuming he was hardened against the EMP, his intangibility should make him relatively resistant to the blast and effects of the nuke.
Wonder Man: Unliving ionic energy, super durable, even if destroyed, he can reform himself over time.
Jack of Hearts: Unliving energy, cannot be destroyed, functionally immortal
Starbrand: Capable of manipulating massive energies directed by the will.
Quasar: Extreme energy manipulation, hard light constructs
Captain Universe: Extreme energy manipulation
Most powerful of known heroic mutants:
Vulcan: A Superman-analog, he was capable of vast energy manipulations and was capable of surviving under almost any conditions.
Shadowcat: It is unclear how long she could maintain her phased state, but while phased she was invulnerable to most forms of physical or energy attack. If she does not actually need to breathe the nuke would be unable to harm her.
Hope Summers: The ability to replicate or duplicate any mutant power she has experienced.
Elixir: Capable of matter alteration, altering living organisms and raising the dead