Ever since she made her debut during the classic John Byrne/Chris Claremont/Terry Austin "Dark Phoenix Saga," Dark Phoenix has been seen as the end all and be all when it comes to epic threats. There is a reason that the story of the Dark Phoenix is being adapted for the second time this century with the upcoming film, X-Men: Dark Phoenix. However, it is worth noting that the Dark Phoenix is slightly less powerful than the Phoenix Force itself. You see, the Dark Phoenix was an attempt by the Phoenix Force to live like a human by copying the essence of its "host," Jean Grey. Thus, it had limitations on it that the Phoenix Force itself does not.
As such, there are a number of comic book characters who could take Dark Phoenix down. We will list 15 of them for you (just from DC and Marvel, as we have a better context of comparison between those two companies that we don't have for characters from other continuities - even Vertigo isn't a fair comparison). Note that we're going to list actual physical beings (or weapons that physical beings could use), not sort of abstract characters like The Living Tribunal or The Presence.
15 THANOS WITH THE INFINITY GAUNTLET
In terms of storylines where comic book characters face off against a being of seemingly infinite power, the only other comic story that could challenge "The Dark Phoenix Saga" would be Infinity Gauntlet, which saw Thanos collect all of the Infinity Gems and use them to power the Infinity Gauntlet, which made Thanos, in effect, omnipotent.
Throughout the series, readers saw those very same abstract concepts that we mentioned like Eternity and the Living Tribunal quake in fear at what Thanos could do to them (and reality itself) with his powerful weapon. Luckily, Adam Warlock was able to get the Gauntlet from Thanos and then split up the gems so that the Gauntlet could not be used again (that, of course, didn't stop future writers from putting the Gauntlet back together a number of times).
14 FULLY-POWERED GALACTUS
Galactus is one of the most difficult beings to quantify, since so much of his power depends on when he has last fed. When he has been without food for a while (and by food, we mean, of course, planets!), he becomes weaker. Therefore, on a good day, there is a chance that the Dark Phoenix could take Galactus down (there was an issue of What If...? that at least theorized that the two might be almost evenly matched).
When he is at full strength, however, he is practically a cosmic force unto himself. Remember, this is the guy who just casually created some of the most powerful beings in the universe in Silver Surfer, Firelord, Nova and Terrax. This is a guy who just has the Ultimate Nullifier lying around in his ship. At full strength, he likely takes down Dark Phoenix.
13 GUARDIANS OF THE UNIVERSE
Similar to how Galactus' power can be seen in how easily he powered other incredibly powered beings like the Silver Surfer, so, too, can the Guardians of the Universe be measured by what they created. After all, they put the Green Lantern Corps together, harnessing the energies that powered the Green Lantern Central Power Battery which, in turn, powers the entire Green Lantern Corps.
That, though, is not the full scope of these tiny blue creatures. Just one of them, Ganthet, was described by Green Lantern Kyle Rayner as being able to destroy a planet with a single punch! IF they all got together, Dark Phoenix would suffer the same fate as the powerful being known as Parallax, who the Guardians of the Universe captured centuries ago.
12 THE BEYONDER/MOLECULE MAN
This one was tricky since these two characters really are like two peas in a pod. It seemed like a waste to give them each their own entry as they basically are the same person. The Beyonder first showed up in the original Secret Wars when he brought a group of Marvel superheroes and supervillains and forced them to fight. One of those villains was the old Fantastic Four villain, Molecule Man, who just wanted to move on with his life.
Through his encounters with the Beyonder, however, Molecule Man moved past his earlier restrictions regarding his powers and he realized that he was just as powerful as the Beyonder. Recently, it was the power of Molecule Man that both saved and then, later, recreated the Marvel Multiverse.
We are mostly shying away from deities in general (it just seems sort of cheap to say, "Who can beat Dark Phoenix? Why, God can!"), but in the Marvel Universe, the Norse gods are so much more than deities, as they are fully formed characters that have been involved in storylines for over 50 years... so we'll count Odin here.
The Jack Kirby drawing we used for Odin's entry should tell you all you need to know about why Odin would win in a fight with Dark Phoenix - he's "the living omnipotence." He's the All-Father, the creator of all things. He has a sword so giant that just taking it out of its sheath could destroy the universe. Ergo, he's powerful enough to take down Dark Phoenix.
10 THE THOUGHT ROBOT
During Final Crisis, the ultimate bad guy at the heart of the story was Mandrakk, the evil Monitor who went crazy (perhaps due to the darkness of the worlds that he was monitoring) and became a sort of vampire-like being, with the "blood" that he was sucking being the very existence of the DC Universe.
The other Monitors built a robot that could defeat him, but it took the energies of two different Superman-level beings to power the Thought Robot, which was piloted by Superman. The Thought Robot was designed to be able to win any fight, no matter what. It practically transcended reality (hence the story being told in 3-D when Superman is piloting the Thought Robot). Superman piloting the Thought Robot was able to defeat Mandrakk, so it seems likely that he would be able to handle Dark Phoenix.
9 DOCTOR STRANGE (AT THE HEIGHT OF HIS POWERS)
While Doctor Strange is currently at a low point, power-wise, in his ongoing series, he has been at many different power levels over the years. When he was introduced, he was "only" a master of the mystic arts. As time went by, he became the Sorcerer Supreme and the number of feats the he pulled off was remarkable. He was present at the Big Bang! He saw the destruction of Earth and everyone but himself die (luckily, the Big Bang restarted everything all over again).
This is a guy who successfully defeated the Dread Dormammu, so he was at a power level where he should easily be able to defeat the Dark Phoenix. However, he never seems to be at full strength, as writers seem to like him not being able to end any given fight right as it starts.
Imperiex appeared early in Jeph Loeb's run on Superman. Superman's old foe, Mongul, showed up on Earth desperate for Superman's help in fighting against Imperiex, who had destroyed Mongul's powerful Warworld planet. Superman teamed up with Mongul and they just barely were able to defeat Imperiex... or so they thought. As it turned out, what they thought was Imperiex was actually just a probe sent by Imperiex to see which worlds he would have to concern himself with when he finally showed up for real.
The real Imperiex cut a swath of destruction through the entire galaxy, killing billions of people on various planets. Eventually, the remaining alien races all banded together at Earth to stop Imperiex, teleporting him through time to the very Big Bang itself! That was their only way of stopping him.
7 THANOS WITH THE HEART OF THE UNIVERSE
Thanos is well known for obsessing with powerful weapons so that he can kill as many people as possible as a tribute to Death. The Infinity Gauntlet was his most famous such weapon, but the most powerful one was the Heart of the Universe. Interestingly enough, the Heart of the Universe was created due to Death. As it turned out, it seemed as though Death was allowing too many people (superheroes, mostly) to return to life and it was throwing the universe out of balance, so the universe created the weapon known as the Heart of the Universe.
Thanos ultimately got a hold of the Heart, and when the Living Tribunal determined that Thanos shouldn't have it, Thanos destroyed the Living Tribunal! He ultimately wiped out the entire universe, only to start things over again, only now with the Heart of the Universe eliminated from existence.
6 MISTER MXYZPTLK
On the one hand, it seems ludicrous to consider Mister Mxyzptlk as a threat to the Dark Phoenix. After all, the mischievous imp is such a buffoon that he is constantly tricked into saying his name backwards and thus having to go back to his home dimension for 90 Earth days. However, don't let his funny exterior fool you. His fifth dimensional powers are practically limitless. Whatever he can think of, he can will into existence.
We saw the extent of his powers when he lent them to the Joker as a joke once and the Joker took over the universe as Emperor Joker. When you can alter reality itself, you most likely will have a good shot at taking down Dark Phoenix. Mxyzptlk could turn Dark Phoenix into, like, a chicken or a bunny rabbit.
In the first volume of the Guardians of the Galaxy (which told the adventures of the original Guardians, who were heroes in the distant future), the heroes came across the young boy who was the head of the Universal Church of Truth (still around from its days in Jim Starlin's Warlock run). The boy, dubbed the Protege, could duplicate anyone's powers that he came across.
Other people have had this power over the years, but the difference is that Protege had no limits (besides having to see the other being use their power first), so he could duplicate the abilities of cosmic beings like the Beyonder and the Living Tribunal. The Protege almost became the new One-Above-Them-All! With a power set like that, Dark Phoenix wouldn't stand a chance against a guy who could just create a duplicate to the Dark Phoenix.
4 ANYONE USING THE MIRACLE MACHINE
The Miracle Machine was originally introduced in the pages of Adventure Comics in the long-running Legion of Super-Heroes feature in that series. The machine was gifted to the Legion and while they appreciated the thought, the dangers of the machine were never far from their minds. You see, the Miracle Machine is powered by thought -- it can turn whatever you think into a reality. As a result, it can easily drive people mad (Matter-Eater Lad tried to get rid of it by eating it once and it just drove him insane).
The Miracle Machine ended up playing a key role in Final Crisis when Superman was given access to the machine and used it to save Earth from Darkseid; in effect, he gave the world a "happy ending" through his wish for everyone.
Another powerful being who debuted in a Guardians of the Galaxy adventure (the original ones, whose adventures were set in the future) was Michael Korvac, who was a human who was mutilated by the evil alien invaders known as the Badoon and had his torso placed on a platform and sent off to fight the Guardians. He gained the ability to absorb energy from other beings.
When he traveled back in time and encountered Galactus, that ability saw himself become extremely powerful by absorbing Galactus' Power Cosmic. It should give you an idea of how powerful Galactus is that Korvac was able to easily kill the Avengers and the Collector (an Elder of the Universe!) using just the power he gained from Galactus. Ultimately, though, he killed himself (but not before first resurrecting the Avengers).
Counting the Spectre for this list and not, say, the Archangel Michael, might seem a bit unfair, since the Spectre is the Spirit of Vengeance and was powered by God. However, the Spectre existed as an independent character for years before he was explicitly connected to a specific religious being. When Jim Corrigan died, he could not get into the afterlife, so a "Voice" turned him into the Spectre. The Spectre had seemingly unlimited power to do whatever it took to achieve his goals as the Spirit of Vengeance.
Oddly enough, the Spectre still had time to join up with the Justice Society of America (you would think that every mission would just be, "Don't worry, guys, I've got this one, as well."). The Spectre is one of those beings that does best with a competent host, as without a host, the Spectre entity goes kind of nuts... sort of like the Phoenix, but WAY more powerful.
1 SUPERMAN PRIME
Do you happen to recall the classic image of Superman moving planets around? That power level was something that was existent during the 1950s, but after Crisis on Infinite Earths, Superman's power levels were significantly reduced. However, in the DC crossover, DC One Million, the Justice League of today traveled 15,000 years into the future to meet Superman Prime, who was actually the modern day Superman who one day entered the sun and lived there for thousands of years.
Now charged up by the sun for over 10,000 years, Superman Prime emerged as the most powerful Superman in history. He easily recreated Krypton (and Lois Lane) and exhibited abilities never shown by a Superman otherwise, so it seems to us like he would defeat Dark Phoenix easily. Superman Prime is, in effect, the ideal superhero... and god?
We bet that there are even more characters that we missed who could defeat Dark Phoenix! Let us know who you think we should have included in the comments section!