When I work with students, I don't promote a goddess from any particular pantheon. During that portion of the instruction, we either explore the gods and goddesses the student's ancestors followed or consider the deities from a specific culture that the practitioner feels particularly drawn to. If a practitioner connects with a specific deity, the deity is normally only called upon in solitary practice. Within group work, instruction, and much of our discussion, we use an archetypical Goddess and God.
The Goddess we focus on is considered to embody the aspects of Maiden, Mother, and Crone, similar to the Christian concept of the Trinity (Father (God), Son (Jesus), and Holy Spirit). In much the same way, the three phases of the Goddess are considered to be held within one deity. We don't consider these aspects to be separate entities and this concept of the Goddess is a powerful tool in understanding personal growth and development cycles.
In magickal work, the Goddess is generally associated with the moon. Certain ritual tools can be seen as corresponding with the Goddess; these are typically circular in shape, are able to hold materials in a womb-like receptacle (like a bowl or round dish), or are curved without sharp lines or angles. For group ritual work, the Goddess is associated with the left hand side of the altar (corresponding with the Western Quarter of the Circle as the altar in the Sylvan Tradition faces North).
As we'll consider a little later, the Goddess follows a natural cycle from Maiden to Mother, from Mother to Crone. The Mother aspect of the Goddess is typically associated with two types of energy. First of all, she is often represented by a pregnant woman or by Mother Earth, corresponding to the Mother's fertile nature, her ability to give life, and her abundance of productive energy. But she is also represented by a Mother who already has children of her own, and in this aspect she is seen as nurturing, caring, strong, intelligent, and growing in wisdom. The Mother is normally associated with the full moon, representative of her abundant energy.
The Crone is associated with the waning moon as her energy is beginning to fade. However, we must remember that at this point, the waning moon has observed the preceding lunar month and has learned from the phases that came before. More than any other trait, the Crone is associated with wisdom. Knowledge is often added to this, but the Crone is normally honored for her wisdom, occasionally in exclusion of other traits (such as perspective and vision, the ability to see cycles and work magick, and the understand of death and the otherworld).
Each aspect of the Goddess can be called upon for the same reason. The key is to cater the way you visualize the different aspects to the strengths of the various phases. For instance, if you were calling upon the Goddess for protection, you could call upon the Maiden as a brave warrior-woman, the Mother's fearless ferocity in protecting her children, and the Crone's sharp wit and cleverness in defeating a challenger with her mind. While each of these can be called on individually, they can likewise be combined into a single visualization as the Goddess is three-in-one.