One possibility, VY Canis Majoris, is so big that if you placed it in the Sun’s position at the center of the solar system, it would extend to Saturn’s orbit!
R136a1 is a good candidate. Scientists recently announced that this star might be some 265 times the Sun’s mass.
For VY Canis Majoris, the habitable zone is between roughly 600 and 1,200 astronomical units (the Earth-Sun distance, or AU) from the star. For R136a1, the habitable zone is located about 2,500 to 5,100 AU from the star.