My favorite part is the final sanding before finish and after all tweeks to tiller are finished. The bow shoots sweet, there's a nice soft touch to freshly sanded wood, it's the part where I connect with the bow (or don't...)
Least favorite is hard to isolate, but some of the things that are in the running would include sanding the sides down on a glass bow, watching a self bow come in below weight again, waiting for wood to dry, waiting for the finish to dry, and making strings.