Since your goal is to soften your hard angles, the best bangs for you are long, side-swept bangs that graze the brows. They should hit just below the brow but be longer on the sides, like these bangs on Jennifer Garner, a natural square face.
The key to your perfect bangs is to ensure that the bangs are tapered on the sides and are longer than they are above the brow. The hair above the brow should hit at the brow or just below it and can be wispier, allowing some of your forehead to peek through.
See my photo gallery of long, side-swept bangs.
Next up: More gorgeous bangs for square faces.