The biggest question of the comic book multiverse is who is the most powerful superhero of all. Some go straight for classics like Superman, forgetting that Kryptonite, no matter how supposedly rare, can disable him. Both Marvel and DC, the biggest players in comic books, have more powerful superheroes than the still-beloved Superman.
Phoenix (Jean Grey)
Jean Grey is known for being close to Charles Xavier, being the girlfriend of Cyclops and being the romantic interest of Wolverine. However, she is also the woman who transformed into Phoenix. She was exposed to massive levels of radiation, but used her existing telekinetic powers to save her teammates.
After Jean Grey's apparent death she became the most powerful of the X-Men and assumed the Phoenix identity. As the Phoenix she is less compassionate, kind and nurturing than Jean Grey. In fact, she is a downright villain.
Phoenix becomes drunk on power, destroying an entire solar system. She eventually kills herself, but the Dark Phoenix comes about later in the series. As with most superheroes, it is never quite certain whether she is gone for good.
Jean Grey is born with telekinetic and telepathic abilities. This is what makes her a good student for Charles Xavier. As Phoenix, these powers are increased greatly. She also gains the power of space flight, resurrection, timeline manipulation, matter and energy manipulation, and more. These powers make her even stronger than Magneto.
Doctor Manhattan (Jon Osterman)
Dr. Jon Osterman was a physicist who was working on an experiment to remove intrinsic fields when he becomes trapped inside the test chamber. He wound up disintegrating, but he reassembled himself as a creature with immense abilities.
Doctor Osterman's first appearances in the month following the accident are of his new physical form's individual systems. For example, his nervous system is the first to appear and then disappear. Finally his entire new physical form appears, proving his attempts to rematerialize a success.
This new being is formed by Jon Osterman and is technically Jon Osterman, but he has abilities that are far beyond any creature in the DC Universe. He is named Doctor Manhattan by the United States government, who use him to intimidate enemies of the U.S. He can manipulate matter, energy and space. He experiences his past, present and future at the same time. He is immortal, because he is pure energy. He cannot be destroyed.
Sentry (Robert Reynolds)
When Sentry is first introduced into the Marvel Universe he is a superhero who has been forgotten by himself and others who knew him. It turns out that Sentry is the one who erased himself from everyone's memory in order to save the Earth from The Void, his evil half.
Sentry has immense powers, first described as the 'power of one million exploding suns.' He can fly, manipulate matter, manipulate energy and manipulate time. He is super strong, fast and agile. He can also heal, mentally project and resurrect himself.