For me it has to be a sheepsfoot or California Clip. I like the sheepsfoot because when cutting line, or dowels, just line it up and give it a whack on the spine. Plenty of strength all the way to the tip for that kind of work. The CA. clip I like for its slightly longer and leaner profile (compared to a standard clip) with a fine point for slicing and detail work.
I have a half congress with the pen blade broken off that is a great little shop knife. I don't like using a pencil for wood working, so this knife gets left out and open for marking wood. I may get around to replacing the blade this year and working it over, but then I would need to find a new shop knife.
-Xander