Thanos, he would bring balance to the world.
I believe most impactful will be Green Lantern Corps.
According to DC continuity, the Green Lantern Corps has been in existence for three billion years, surviving multiple conflicts both internal and foreign. Currently operating divided as pairs amongst the 3600 "sectors" of the universe, there are 7202 members (known commonly as Green Lanterns)
Green Lantern Corps oath:
In brightest day, in blackest night,
No evil shall escape my sight.
Let those who worship evil's might,
Beware my power...
Green Lantern's Light!
Why not others:
- Flash will at some point mess up the timeline like he always do,
- Batman is amazing but he cannot be that symbol of hope for the civilians, (though he is bad-ass :p )
- Superman's existence will create huge chaos in citizens mindset (exactly like they have shown in Man of steel )
(Ask away in comments if you have any other contender)
The thing is, very few superheroes are written as if they actually want to change the world. Mostly, regardless of how powerful they are, all they succeed in doing is making stuff a little better locally until the next story arc begins.
So the obvious first choice of a hero who has real impact is Miracleman. He replaced the terrestrial governments, institutes significant improvements and ushers in a new era in which anyone can raise a superpowered child.
Except… the unfinished Neil Gaiman arcs were going to be called the Silver Age and the Dark Age. So maybe it doesn't turn out so well.
Maybe the answer is codename V. There are certainly a number of current governments who should be afraid of their people. But I am not sure we are ready for the anarchy, in the truest sense, that is V's stated goal.
Maybe Ozymandias would be a decent option. The world isn't quite as bad as the one he faces in Watchmen, so perhaps the world's smartest man could make a difference in a less… violent… way.
Moving away from the works of the Great Wizard of Northampton, I thought about Warren Ellis's Planetary or The Authority, except they are constantly at loggerheads with governments and other existing power structures. I rule out the team behind Injection, because they made things worse but haven't yet made them better. So then I thought about Miranda Zero of Global Frequency, who would be a very fine choice, if a bit localised and troubleshooty in her actions.
And she would have remained my choice, but then Robert Kirkman gave us the final issue of Invincible, which is the object lesson I didn't know I was waiting for. Mark Grayson's future isn't without conflict, but there is an underlying optimism and an impact that isn't constrained to Earth but spans the entire galaxy.
If you haven't read it yet, Invincible is a comic I can't recommend highly enough.
Professor X. He could make us all happy.
Captain America. He is a true patriot
Surely not Batman: he would have done much more good if he had invested the time he used as a vigilante in his company.
Wolverine. He is the ultimate dream of scientists. Regeneration and indestructibility. How much progress would we have if scientists discover a way to create a wolverine? It will be the peak of human innovation and it could also be the start of its fall.
We would all be drunk and happy all the time.
Flash. I feel like most of the other superhero's powers can be replicated with money.
I believe that the world would be much better if superman lived among us. A lot of disasters, wars, crimes and violence would be avoided. Probably that would decrease too much the motivation of some people to break the laws and certainly his values would also serve as example for good people.