By far it's the planning for me. I enjoy researching the prop then figuring out how to make it happen. (But I guess that's the engineer in me.) I also like sketching it up which satisfies my artistic side.
The part I like the most is when my daughter (9) and son (6) get excited showing off a new prop I've built to other people. It's a combination of the excitement in the voice and the pride in their eyes.
The actual building of the prop is probably my favorite part but I also get a lot of satisfaction out of seeing the end result. Then there's a whole other level when you put everything out for display and hear all the comments from visitors. I just love the whole process really.
Hard to which is my favorite, I guess all if it; from the design phase to garage sales to building walls (well maybe not building walls) set decorating, cranking up some hard rock and getting in the zone.