On my PC I use Corel Draw, on the iPad Autodesk Sketchbook, Touch Draw, or the Notes application, on the Surface its usually Sketchbook but I've also got Sketchable, and on Linux Inkscape.

The worst thing is the linkage between knowing what you want to do and assembling the right tools to do it in the app you're looking at. Most of my drawing is technical in nature (systems, small components, software architectures, Etc.)