http://en.wikipedia.org/wiki/Superpower
A superpower is a state with a dominant position in international relations and is characterised by its unparalleled ability to exert influence or project power on a global scale. This is done through the means of both military and economic strength, as well as diplomatic and soft power influence.
Basically, you need wealth, cultural influence, a very good military, and good international relations.
If it were up to me, I'd also include scientific advancement, so feel free to say something about that too.
There are many, many, many candidates, but in order to narrow the votes and to have a clearer view, I won't put all the developping countries/advanced countries in the poll, just those we are talking about right now.
The United States of America - A Superpower since nearly a century, has bases all over the world, and maybe the greatest cultural influence one can have.Although slowing down in both scientific and economic power since the 1980s, it's still ahead of many countries.It's a key nation in world diplomacy.
I personally think that although some people seem to "freak out" about how all empires fall, this one is made to last - Namely because it went through a very bad crisis already, in 1929 to be exact, and it has slow but adequate growth both in terms of population and economy.
One thing that I fear tho is the massive scale of agriculture (=Damaging land) and how soils are polluted by shale gas.
China - It is becoming a key nation in Asia, however their population (More males than females, and aging population), lack of cultural influence and modernization will hold them back.
India - It has a very strong population but not so much military influence.I'd say mild for culture, but I doubt all the Buddhists over the world would one day emigrate there.
As a side note, don't expect a strong OP presence, I'm not very active on the forums but I'm interested in seeing what you think.