Depends on how you define power.
If we take it as transferring energy then it will have to be the most luminous.
So the most luminous star known in the Universe is R136a1
He-Man might be the most powerful man in the universe and he definitely has the reputation of being very powerful. Superman is also considered to be one of the most powerful men in the universe, so this is a matter of some debate.
God, who created the universe.
Hyperstorm
a black and white hole
No one knows for sure. No one has a telescope powerful enough to see every star in the universe
A Super Nova, exploding star, is by far the most destructive thing in the universe.
The spike hammer or the fantastic pilum.
Known Universe - 2009 Most Powerful Stars 3-3 was released on: USA: 19 May 2011
As the question specifies brightness, the brightest star in the Universe is our Sun - because it is the closest star to us. The most luminous star is another question: See related question.
The cosmic cube is the same as the teserract. They are both the 2nd most powerful thing in the universe. The most powerful being in the universe would be galactus because he is older than the universe itself and absorbs planets to give him even more power.
star cannon
New York City, the Center of the Universe.