Which gas has the greatest volume?
First of all, if in a certain condition it is said that mass (or any other parameter) is constant, you cannot just assume ${1gm}$ (or any other number). Mass being constant means $\Delta m =0$, not that it is ${1gm}$. You have no way of knowing what the mass is (unless stated in the question, of course). The correct approach is to assume the mass to be $'m'\space{units}$. Now, this mass $'m'$ can assume any value, according to the condition.

Moving on, you have made a mistake while comparing molar mass and volume.

The ideal gas equation gives us the relation,

$$PV=\frac{\text{Mass}}{\text{Molar Mass}}RT$$

For your question,

$$V=\frac{1}{\text{Molar Mass}}.\frac{(\text{Mass})RT}{P}$$

$$V\propto \frac{1}{\text{Molar Mass}}$$

Recheck your calculations.