I have seen numerous different shapes for a menorah. The simplest ones have candles in a straight line. Many have curved branches. The only general requirement that I recall reading in Kitzur Shulchan Aruch (139) (thanks to Ezra for info, on different question) is that the candles must be in a straight line.
I have noticed that all the Chaba"d menorot have the branches emanating from a central post in a diagonal fashion, like the letter V. Why does Chaba"d use only this shape?
I gather that there are a number of interpretations of what the shape of the Temple menorah looked like. In your answer, along with citing the source, explain why they follow this interpretation.