It does depend on your customers a bit - I have been involved in PTA and local bake sales.
Bestsellers are the classics: chocolate cake, victoria sandwich, most big layer cakes eg coffee and walnut where it's immediately clear what flavour it is. They are a bit harder to serve and look nice, though
millionnaire's shortbread - always disappears fast!
Also brownies and lemon drizzle cakes quite popular.
For kids, anything with something on it, eg school cake with smarties/marshmallows on, choc cupcakes with a mini egg / choc orange slice etc on it
Unpopular: ginger biscuits, any biscuits that aren't chocolate, flapjack, banana bread, plain scones kind of thing. Too plain and not enough of a treat (I'm guessing)