Order of electrophilic substitution
In your series, all of the alkyl benzenes will have roughly the same +I inductive effect.  Where they differ is with regard to the resonance effect.  Hyperconjugated structures such as those drawn below for ethylbenzene are often invoked to explain these differences.



There are two hydrogens in ethylbenzene that are capable of donating electrons into the aromatic ring by hypercojugation.  In isopropyl benzene there is only one and there are none in *tert-*butylbenzene.  Hence ethylbenzene should be the most electron donating of the compounds in your series due to resonance involving hyperconjugation.  

Realize that these effects are relatively small, for example the relative rates for the electrophilic nitration of various aromatic compounds is:

benzene=1, toluene=24, tert-butylbenzene=15.7 (reference, page 1060)

Benzene has no substituent (other than hydrogen), so neither resonance or inductive effects play a role, it would be the slowest.  The expected order in your series would therefore be:

ethylbenzene > isopropylbenzene > tert-butylbenzene > benzene