When does a gas deviate the most from its ideal behaviour?

For real gases,the relation between $p$,$V$,and $T$ is given by the van der Waals equation:
$(p + \frac{an^2}{V^2})(V - nb) = nRT$
Where $a$ and $b$ are van der Waals constants,$nb$ is approximately equal to the total volume of the molecules of a gas,and $a$ is the measure of the magnitude of intermolecular attraction.
$(i)$ Arrange the following gases in the increasing order of $b$. Give reason: $O_2, CO_2, H_2, He$
$(ii)$ Arrange the following gases in the decreasing order of magnitude of $a$. Give reason: $CH_4, O_2, H_2$

At $S.T.P.$,if the compressibility factor $(Z)$ of a gas is less than $1$,then which of the following is true?

Helium is suitable for gas thermometry at low temperatures because of its ...

At very low pressure,if the molar volume of $CO_2$ is constant,its compressibility factor is ..........

The values of van der Waals' constant '$a$' for gases $O_2, N_2, NH_3$,and $CH_4$ are $1.360, 1.390, 4.170$,and $2.253 \ L^2 \ atm \ mol^{-2}$ respectively. The gas which can be most easily liquefied is