The cut depends on the stone. If the stone has fire, then a round. For other stones I tend toward trillions, pears and the pregnant marquise (also called the onion cut). Quantum cuts are fantastic for squares. Emerald cut for emerald and tourmaline.
For the life of me, I can't figure out why anyone would put an Asscher cut on a black stone like spinel or tourmaline. But, they do.