Nails.
Nails give you the best return of money for the time it takes to produce.
Whenever I have the Building Supplies Store idle, I set it to produce nails. Even when busy doing other things elsewhere, I can leave one factory free to produce metal, to feed the Building Supplies store.
I find other stores like the Hardware Store and Farmers Market are often too busy - I tend to need a lot of production here, for Contest of mayors assignments and such. Anyway, it is easy to interrupt nail production to get out a couple of planks or something. I don't have to wait too long, even if I have the whole production chain full. (I have 6 slots)
I guess I like to keep things simple, and it doesn't get much simpler than nails. 🙂
