Machining? Sheet metal? Injection moulding? Die Casting? etc I enjoy designing for any process, but I think I'll say machining here - it's relatively straightforward almost theraputic to design for , you can get fast results and parts usually look great! I do however like to design for a process I've never worked with before (e.g. I designed a deep-drawn part for the first time recently). I also enjoy desiging die-castings and injection mouldings but these are much more difficult and time consuming to get right.