I have to admit that one of the most reliable and versatile tools I know is a humble ballpoint pen. Even though I enjoy all sorts of crazy and cool fountain pens, spend hours finicking them into perfection only to start all over on the next day, still, if you ask me what one drawing tool will be with me on the desert island - my answer will probably be a ballpoint pen :) Though there are some ballpoint pens I like more than others ;)