When it comes to the shape of diamond, the right shape to buy is the one your woman wants... period. Do not worry about which shape retains it's value or even which one is the most brilliant. The bottom line is this. What your girl likes is what she likes and it is her personal taste that comes into play when deciding what shape to get. All the different shapes fall into either one of two categories. Brilliant cut or step cut. Only one stone, the emerald cut is called a step cut, the rest are brilliant cuts. Hence a round diamond would be properly called a "round brilliant cut", pear = pear brilliant cut, etc.
Value in Shape
On the diamond market there are some shapes that are more valuable than others and contrary to what you've been told, here they are in order of their value. Supply and demand has a big role to play here.
marquise, pear & oval
step and brilliant cut squares and rectangles
heart & trillion
Surprisingly there are a lot of jewelry stores that teach the opposite and sell fancy shapes for more money than they do rounds. As of this publishing, this buyer has never encountered common fancies that were more expensive than rounds. Also keep in mind that these are engagement ring size diamonds, ½ct and up. When it comes to smaller diamonds that go into the sides of rings (called melee) the order would be reversed, with rounds being the least expensive and fancies being more expensive.
Here are some of the more popular shapes that diamonds come in today.