In the gas phase, which of the following alcohols is the most acidic?

In the gas phase there is no solvent, so the larger and more polarisable alkyl groups stabilise the alkoxide anion better by charge dispersal. The order is $(\mathrm{CH}{}_3){}_3\mathrm{COH}>(\mathrm{CH}{}_3){}_2\mathrm{CHOH}>\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{OH}>\mathrm{CH}{}_3\mathrm{OH}$, the reverse of that in water, where solvation dominates.