There are trillions upon trillions of stars in the Universe. Some are only a fraction of the Sun's mass, while others are equivalent to hundreds of Suns. It is important to note that "most massive" doesn't necessarily mean the largest. That distinction depends not only on mass, but at what stage of evolution the star is currently in.
Side Note: Current theory constrains a star's mass to about 120 times that of our Sun. Beyond this, we don't understand how a star remains stable. Yet, the stars at the top of this list are beyond that limit.
Without further ado, here are the 10 most massive stars in the Universe.
The Wolf-Rayet star R136a1 currently holds the record as the most massive star known to exist in the Universe. At more than 265 times the mass of our Sun, more than double most stars on this list, researchers are somewhat baffled at how such an object can even exist. It is also the most luminous at nearly 9 million times that of our Sun. It is part of a super cluster in the Tarantula Nebula, which is also the location of some of the other massive stars of the Universe.
2. WR 101e
The mass of WR 101e has been measured to exceed 150 times the mass of our Sun. Very little is known about this object, but its sheer size ears it a spot on our list.
3. HD 269810
Found in the Dorado constellation, HD 269810 (also known as HDE 269810 or R 122) is nearly 170,000 light-years from Earth. It is about 18.5 times the radius of our Sun, while outputting more than 2.2 million times the Sun's luminosity.
4. WR 102ka (the Peony Nebula Star)
Located in the constellation Sagittarius, the Peony Nebula Star is a Worf-Rayet class blue hypergiant, similar to R136a1. It may also be one of the most luminous stars, at more than 3.2 million times that of our Sun, in the Milky Way galaxy. In addition to its 150 solar mass heft, it is also a rather large star, some 100 times the radius the Sun.
5. LBV 1806-20
There is actually a fair amount of controversy surrounding LBV 1806-20 as some claim that it is not a single star at all, but rather a binary system. The mass of the system (somewhere between 130 and 200 times the mass of our Sun) would place it squarely on this list. However, if it is in fact two (or more) stars then the individual masses could fall below the 100 solar mass mark. They would still be massive by solar standards, but not up to par with those on this list.
6. HD 93129A
This blue hypergiant also makes the shortlist for the most luminous stars in the Milky Way. Located in the nebula NGC 3372, this object is relatively close compared to some of the other behemoths on this list. Located in the constellation Carina this star is thought to have a mass around 120 to 127 solar masses. Interestingly, it is part of a binary system with its companion star weighing in at a not-insignificant 80 solar masses.
7. HD 93250
Add HD 93250 to the list of blue hypergiants on this list. With a mass about 118 times the mass of our Sun, this star located in the constellation Carina is about 11,000 light-years away. Little else is known about this object, but its size alone earns it a spot on our list.
8. NGC 3603-A1
Another binary system object, NGC 3603-A1 is about 20,000 light-years from Earth in the constellation Carina. The 116 solar mass star has a companion that tips the scales at more than 89 solar masses.
9. Pismis 24-1A
Part of the nebula NGC 6357, located in the Pismis 24 open cluster, is a variable blue supergiant. Part of a cluster of three nearby objects, 24-1A represents the most massive and most luminous of the group, with a mass between 100 and 120 solar masses.
10. Pismis 24-1 B
This star, like 24-1A, is another 100+ solar mass star in Pismis 24 region within the constellation Scorpius.