Which carbocation is the most stable?

Stability increases with the number of alkyl groups (inductive effect and hyperconjugation). The tertiary cation $(\mathrm{CH}{}_3){}_3\mathrm{C}{}^{+}$ is more stable than secondary $(\mathrm{CH}{}_3){}_2\mathrm{CH}{}^{+}$, which is more stable than primary ions.