Which of the following carbocations is the most stable?

The allyl cation $\mathrm{CH}{}_2=\mathrm{CH}-\mathrm{CH}{}_2{}^{+}$ is stabilised by delocalisation of the positive charge over two carbons (two resonance structures). The other three cations have no resonance, and their stability falls as the number of alkyl groups decreases. Hence the allyl cation is the most stable.