Yes iron is the best, but demand does drop off. I have over 20 selling slots, so I used to make iron with all factories till sales slowed down then I would switch to wood, until that slowed down, then I would switch to plastic, which allows a little time to click bubbles or go on the gthq as soon as I had room I would switch back to wood. You can easily make 20k per hour but it is a little boring.
