The stone should be classy and of above par cut, as well as brilliant and fiery. This entails the use of competent skill by the cutter because that majorly affects the proportions and symmetry, which in turn affect the sparkle.
Heart-shaped diamonds hold a special place, in that they are often more sentimentally valuable. You can have one set as a solitaire or with side stones. Diamonds of this shape are hard to get under 1 carat, and it may be that you cannot afford this option.
Take your time researching what is available inside your budget, and make an informed decision after inspecting the stones you take a liking to. Remember though, that the heart shape, like the pear, is not as common as the other shapes. Consider that before dismissing a stone for the simple reason that it holds a minor flaw.