A good shape for a musical melody often resembles an arch. This is because an arch shape can create tension and release, building up to a peak and then descending, which can keep listeners engaged. It allows for both variations and emotional highs and lows, adding dynamic interest to the melody.
However, elements like zigzag can also be effective in creating excitement and movement, often used in faster passages or to convey urgency. Ultimately, the best shape may depend on the style and emotional intention of the piece.