Maximum malignant potential is in -

A junctional nevus is a mole that occurs between two layers of skin. This image shows two junctional nevi that appear as uniformly brown small macules, round in shape with smooth regular borders It often requires a dermatologist to fully evaluate moles. For instance, a small blue or bluish-black spot, often called a blue nevus, is usually benign but often mistaken for melanoma. Conversely, a junctional nevus, which develops at the junction of the dermis and epidermis, is potentially cancerous. Ref Harrison20th edition pg 1234