Which alcohol is dehydrated most rapidly by dilute acid at the same temperature?

The rate-limiting step forms a carbocation, so the fastest dehydration is the one giving the most stable cation. The triphenylmethyl cation is stabilised by resonance over three rings, more than the tert-butyl cation (hyperconjugation), then $(\mathrm{CH}{}_3){}_2\mathrm{CH}{}^{+}$ and then ethyl.