I'm a sketchbook person. I do my best design work while traveling, where I have fewer distractions. In past years I've carried Moleskine sketchbooks, pens and watercolor pencils. Recently, I've been experimenting with utilizing the iPad that always accompanies me.
When you open the app, you see a collection of sketchbooks, which closely resemble those Moleskine notebooks in my collection, but with the option to customize the covers. Each one opens up, looking just like the physical thing, and allows you to flip through the pages. Once you find a page to work on, you can go to full screen and begin working with the fountain pen and erase tools. To get more flexibility, you can purchase additional inexpensive tools, including a pencil, pen, marker, and paintbrush, each of which does pretty much what it sounds like it should. The easy undo/redo is a big bonus. The paintbrush is still undergoing some evolution to get more lifelike control (a color mixer tool was just added), but still I like it. I find that a Sensu capacitive brush/stylus gives me the best control.