WARNING: The following contains spoilers for Marvels X #4 by Alex Ross, Jim Kreuger, Well-Bee, and VC's Cory Petit, on sale now.
With her powers of probability and reality-altering, Scarlet Witch is undeniably one of Marvel's mightiest Avengers. For years after she debuted in Stan Lee and Jack Kirby's X-Men #4, Wanda Maximoff was in the running for the title of Marvel's most powerful mutant. However, the Scarlet Witch is no longer considered a mutant, because her abilities were revealed to be the work of the High Evolutionary in modern Marvel Universe continuity.
However, the Scarlet Witch is still considered a mutant in many of Marvel's alternate universes, where she ranks alongside powerhouses like Jean Grey, Professor X and even Magneto. In Marvels X #4, however, her title as the most powerful mutant in the world is stated unquestionably by fellow Avenger Iron Man. While this might seem like the friendly embellishment on Tony's part, Wanda Maximoff's status as a powerful mutant still holds true in the world of Earth X.
The issue centers around Spider-Man and Daredevil trying to rescue a human named David, who may be the only merely human boy left in a world where the Terrigen Mists have given every human powers. David had been captured by Kraven the Hunter, who himself had been ironically mutated into a humanoid lion. Seeing him as the ultimate quarry, given his potential status as the last normal human, Kraven plays with his prey much in the same way that he had for years with Spider-Man.
Thankfully, Captain America and several other costumed heroes appear, keeping an armed David from making a mistake which he might regret. After taking Kraven down with ease, the Fantastic Four and the Avengers are introduced by Iron Man. With the exception of more modern members such as Spider-Man and Luke Cage, the team resembles its more iconic lineup, complete with Avengers stalwarts like Hawkeye and Vision.
Part of that lineup is the Scarlet Witch, who Iron Man mentions is the most powerful mutant alive. Even if she's no longer a mutant in the main Marvel Universe, the Scarlet Witch is still considered one in the world of Earth X, which is an extension of an era where the Scarlet Witch was still a mutant.
Marvels X is a prequel that directly leads into Earth X, a Jim Kreuger and Alex Ross project that essentially functioned as Marvel's equivalent to DC's Kingdom Come. The premise had the world's human population suddenly mutated by a mysterious plague, which was itself was caused by the release of the Terrigen Mists. The villain of the story was the Skull, a psychic powered teen who could coerce others to do his bidding. The extent of his unrealized power was so immense that when these powers were first activated, the energy blowback killed the rest of the world's most powerful psychics. This included the X-Men's Professor X, whose death has long since transpired by the events of the series. Given that the Phoenix Force eventually shows up in the land of the dead, it is suggested that Jean Grey is dead as well. Other powerful mutants, such as Emma Frost, are unaccounted for, and a shift in the world's polarity robs Magneto of his powers.
This power vacuum of mutants definitely leaves the door open for Scarlet Witch to be the most powerful member of their race. Unfortunately, this extreme power is apparently not enough to protect Wanda, or any of her teammates, for that matter. By the time of Earth X, by which point Iron Man is incredibly old, Stark leads an army of Iron Avengers. This is because many of the original heroes, including Scarlet Witch, have been long dead, with the only remnants left being these robotic facsimiles.
It's explained that between the events of Marvels X and Earth X, Absorbing Man copied the powers of many of the Avengers, killing them until Vision did the same to him. Before the android could do this, however, Absorbing Man created a reverse hex equation that killed Wanda by snapping her neck.
Despite that dark, doomed destiny, the Scarlet Witch still stands as one of the last powerhouses of the mutant race in a world that grew stranger than almost any Marvel Universe before it.