Umm, Magneto can only control metal. Your blood isn't metal. Jean Grey on the other hand, now she can turn you into dust.
At the risk of going ultra-geeky: Magneto controls electromagnetism, which is not just effective against metal.
http://www.magnetowasright.com/pages/analysis/the-science-of-magneto.php
However, the iron in your hemoglobin thing was a step beyond by the writers as iron loses its magnetic properties when it bonds with oxygen.
Anyway. Dr. Manhattan.
Unbound by concerns of the physical world, can live outside of time, cannot be harmed, can call anything into existence by willing it and can even be in several places at once. And can turn Superman, Batman, Wolverine, Deadpool, Goku, Galactus and Rorschach into a rapidly expanding bubble of goo without even thinking about it.
I think that qualifies him as "best".
I would agree but he is easily confused by tachyons and random chance/fate.
However, I must also argue against the superhero title. Metahuman (if he can be classified as such), yes, superhero, no. There is a certain moral obligation to the hero title and he lacks the moral conviction necessary due to his being unconcerned with the physical world. If your reaction to the extinction or mass murder of a sentient species is merely boredom then I have trouble considering you a hero. That is compounded even more if your reaction to finding the man responsible is to decide he is right.
My all-time favorite was always Wolverine. The healing factor is, or was, a bit too much. In the Civil War storyline Nitro incinerated Wolverine to a skeleton, and he healed. Deadpool has lost his head and had it heal when reattached to the neck. After Civil War they actually worked to slow down Wolverine's healing factor a bit. However, it can be overtaxed so that he is down and out for an extended period of time, as he was when Magneto ripped out his adamantium.
And those with healing factors do age, although Deadpool was seen by the New X-Factor 800 years in the future and in Wolverine: The End Logan has clearly aged, has one broken and unhealed claw, although he still exhibits a healing factor, and mention is made of him being 210 years old, but that was from another character's quote and not a statement of fact. I doubt its validity considering that Wolverine: Origin is in the same timeline/reality and has him being born 200+ years ago. At best guess, I would say he was closer to 400 years old in The End.