One way to approach this, Matthew, is to select your submissions so as to demonstrate as thorough a knowledge of the basics as possible. You might review
this Cafe thread, in which some members listed the most important features they looked for in a good painting . Then consider which of your drawings or paintings tend to exhibit those qualities to the greatest degree.
The caveat here, I suppose, is to be careful about clinging to a "favorite," because, say, the eyes turned out well, if in fact the rest of the drawing, design, composition and the like aren't pretty close to being equally demonstrative of your artistic appreciation for the piece as a whole.
Of course, you can only submit what you have. Give it your best shot, and then try to be open and flexible to adjust and adapt to whatever eventuates. (Why, this year alone I will fail to place in either the PSA or ASOPA competitions, and yet I just keep on truckin.)