It has the lowest absolute magnitude, which means it has the highest intrinsic brightness. B stars are the second hottest type of stars, and the numbers within each type increase with decreasing temperature. So a B1 star is hotter than a B7. G2 type star and a main sequence star, so most like the Sun. Antares, because it is spectral type M1, which means it's a cool, red star and because it's luminosity class I, meaning it's a supergiant.
Molecular clouds are colder and denser than the rest of the interstellar medium.
Because they are cold, the thermal pressure is low , which means that in the densest areas, gravity can overcome the thermal pressure and start the collapse process that eventually leads to star formation. The fact that molecular clouds have molecules does not mean that they'll form stars. Molecules form because it's cold and dense, but it's because the cloud is cold and dense that stars form. One had nothing to do with the other, they just have the same reason for being.