The notched lapel (also known as the step collar or the step lapel) is perfect for single-breasted men’s suits. The notch is V or an arrowhead shape basically pointing in towards the notch on the opposite lapel; it’s a versatile and quite conservative style, while the other two lapel styles – peak and shawl – are very distinctive and a bit showy.
A really small notch lapel is called a “fishmouth”. It is distinctive, but does require equally small accessories like pocket squares.