Depends on what you are trying to do and the stage of the game in my opinion.
Start up, you target the farm so they have a hard time rebuilding.
Mid-late game. Will depend on the goal of the op. If you are trying to noble the guy targeting a level 30 farm really isnt going to help you. Might knock it down to 28 but you plan to noble the village anyways. In my opinion in this case, you are better off targeting the wall to make sure the wall is zero when the nobles hit. If you are just nuke opping him so he doesn't support his tribe mate you are really opping then you go to the farm. You might plan to noble him in the future but for now if he dodges he can't do anything because he has to many troops for the village and if he doesnt dodge he noble has no troops and he has to waste 400k in res just to build farm back to level 30. You do this to most or all of his villages and it because a hassle.
This is my opinion anyways.
~ Happy
