If the diamond is an engagement ring, it should probably be in a shape that is considered classic, that won't get tiresome to look at. Probably the classic round cut, marquis, emerald (squared off shape) would be preferable to a butterfly or heart shaped cut which would lose its novelty after awhile.
The bottom line is preference. My favorite is marquis, personally. I have a small round cut diamond which glitters beautifully, and since it was given from the heart, I am more than satisfied with it.