Which number representation takes the largest amount of memory?
The question seems rather bizarre. 

Using n bits, each representation can represent $2^n$ different values. Two of them have different representations for +0 and -0. I would assume that signed-magnitude and one's complement are used if someone values the ability to distinguish +0 and -0. 

If the question was "which representation takes the largest amount of memory", then that question is already nonsense because no matter what you think about +/- 0, one's complement and signed magnitude are the same. If the question was "which representations take the largest amount of memory", then I'd still say that claiming one (or two) take more than the others is very misguided. 

(And the number $-2^{n-1}$ that can be represented with n bits two's complement or signed magnitude has its own problems where both negation and division can lead to overflow).