"Reverend Smith", or whatever their surname may be. I don't care how common it's become in the last few decades, it s as wrong as "Sir Smith". Both Rev. and Sir have to be followed by a first name, or at least an initial, and Rev. can be followed by Mr/Mrs/Miss/Ms, if you want to be formal (eg the Reverend Mr Smith). Furthermore, "Rev." isn't even necessary: "Mr (or Mrs etc.) Smith" is perfectly correct. The Methodist ministers in the church I grew up in were usually referred to as Mr Hoyles, Mr Bellwood, etc. (In our less formal age, they'd probably be Arthur and Arnold.)