Electrophilic alkylating agents deliver the equivalent of an alkyl cation.  Alkyl halides are typical alkylating agents. Trimethyloxonium tetrafluoroborate and triethyloxonium tetrafluoroborate are particularly strong electrophiles due to their overt positive charge and an inert leaving group (dimethyl or diethyl ether).  Dimethyl sulfate is intermediate in electrophilicity.