The rectangular face shape is a strong looking face shape due to the square jawline.
So to obtain harmony you should look for glasses in the more soft shapes like round glasses. Also the panto shape which is a round but not circular shape will look good on a rectangular face. If you wish to stand out a bit more you could try an oval frame.
Oval frames are not for everyone but for those who like them they add a very distinguished and elegant touch to your appearance.
For the feminine audience a cat-eye shape is also good on the rectangular face shape.
For the sake of harmony you can try to find glasses where the upper frim of the frame follows your eyebrows.
READ MORE ABOUT FACE SHAPES AND GLASSES HERE