It is not easy to know which is the brightest star in the universe, because all depends on where that star is located if we look at it from Earth. In our sky, the brightest star is Sirius. It is sometimes called the Dog Star and it always flickers with many colors.

If we want to think about "the brightest star in the universe", scientists need to know the location of the stars, because they might not look very brilliant to us, but it is because they are very far from Earth. Suppose we could put all stars at the same distance from Earth, then which one would be the brightest? Well, we don't know which star truly is the brightest, but we know that some of the stars are remarkably bright. For instance, there is one called Deneb. This star forms part of the three stars on the group called the Summer Triangle. Deneb is farther from Earth that other stars that we can see, and it is around 100,000 times brighter than the Sun.

You can "click" on the links that you see in this answer, and find out pictures and more information about "Sirius" and "brightest stars".

I'm not sure we know what the brightest star in the entire universe is, because whether we can see the entire universe clearly is still a subject under research. The brightest star to us on Earth is of course the sun, because we're closest to it. If we rate stars based on actual measurements of brightness that also considers distance from the Earth - because naturally the farther away the star is, the dimmer it will look to us even if it is very bright, so we have to put the distance from Earth into the measurements, too, to "correct" for what looks dim to us - then the brightest star we can measure right now is called Sirius, and it's in the Canis Major ("Greater Dog") constellation.

Hope this helps.