There are only three constitutional isomers (see the image below).
Ethyl methyl ether is the least acidic because it has only C-H bonds. However, it should be more acidic than an alkane (pKₐ ≈ 50) because of the electron withdrawing effect of the O atom. I would expect it to have pKₐ ≈ 30.
The alcohols are more acidic because the O in the O- H bond is highly electron withdrawing. Alcohols typically have pKₐ ≈ 17.
Alkyl groups are electron-donating. This increases electron density in the OH bond and makes the acid weaker. Propan-2-ol has two methyl groups on the α-carbon, while propan-1-ol has one ethyl group. Thus propan-2-ol should be a slightly weaker acid than propan-1-ol.
