In the gas phase the correct order of basicity of methylamines is:

In the gas phase there is no solvation, so only the +I effect of the alkyl groups matters, and basicity increases with the number of methyl groups: tertiary > secondary > primary > ammonia. The order $(\mathrm{CH}{}_3){}_2\mathrm{NH}>\mathrm{CH}{}_3\mathrm{NH}{}_2>(\mathrm{CH}{}_3){}_3\mathrm{N}>\mathrm{NH}{}_3$ is the order in water.