If this star--R136a1 by name--were where our Sun is, its apparent magnitude would be fourteen whole magnitudes brighter than the Sun's. Gulp.
The star is in RMC 136a, an open cluster inside the Tarantula Nebula, which in turn is in the Large Magellanic Cloud, 165,000 light-years away. Its current mass is ~260-270 solar masses and an original mass of up to 320 times that of the Sun.
More information at:
http://www.ras.org.uk/news-and-press/157-news2010/1867-300-solar-mass-star