I am a mechanical pencil snob, and have a ton of different options. One I really like to use with my Woodpeckers t-square (since it has the little holes you can run the pencil in and slide the square down your board), is the Pentel Orenz Nero 0.5mm. It's expensive and has a lot going on, you also have to learn how to use it without the lead coming out of the end, but it just pleases me so much to use. I also have several 2mm lead holders, Caran d'Ache Fixpencil are my favorite (basically a lead holder version of the 849 ballpoint pen). Last, my favoritest favorite, is a solid brass lead holder from Ystudio. It's heavy and just makes me ridiculously happy to use in woodworking or just drawing with it. But that's a $100 mechanical pencil so not for everyone, of course.