Cubed/rectangular. Let me explain.

There are many things that make a watermelon perfect. Taste, Texture and Temperature.

Taste must be sweet and fresh, many watermelons accomplish this.

Temperature - slightly cold, but not frozen, freezing a watermelon completely ruins it, no matter how sweet it is.

Texture - slightly hard but easy to bite into, this is where shaped slices come into play. Because your mouth takes a more rectangular shape, when you have cubed/rectangular watermelon slice it can fit your mouth perfectly, the left side reaching the left side of your mouth, the right side reaching your right. The top of it touched your top teeth when you bite, as well as the bottom touches your bottom teeth. This is why cubed/rectangular watermelon slices are the best. Triangle pieces are irregular and would be an uneven fit in your mouth.