I would have to say mechanical products are the most challenging and educational because you get to define materials and why as well design with all types of functions, aesthetics and ergonomics for all kinds of requirements and conditions.

I was also fortunate to have had more than a few opportunities to design simple machines and even some automation.

As well I've done a few firsts when I got to redesign some complete machine lines for cost reductions from material selection all the way through the process working with cut and prep, welding, machining, painting, assembly, testing, break down and loading the containers.