Which of the following allylic cations is the most stable?

Each allylic cation is a hybrid of two structures. In $(\mathrm{CH}{}_3){}_2\mathrm{C}=\mathrm{CH}-\mathrm{CH}(\mathrm{CH}{}_3){}^{+}$ the two contributors are $(\mathrm{CH}{}_3){}_2\mathrm{C}{}^{+}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}{}_3$ (tertiary) and $(\mathrm{CH}{}_3){}_2\mathrm{C}=\mathrm{CH}-\mathrm{CH}{}^{+}-\mathrm{CH}{}_3$ (secondary), so both termini are alkyl stabilised. In the others, one terminus is primary or the cation has fewer alkyl groups, so they are less stable.