Which carbocation is most stable?

A phenyl group, in which there is a flow of electron due to resonance of the benzene ring, generally causes -I effect, but truely speaking it can act as both electron donating as well as electron withdrawing groups, depending upon the nature of group with which it is attached. In $\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2^{+}$, phenyl group is attached to electron deficient group, hence it will donate electron due to resonance in benzene ring and thus stabilise the cation.