If you are like me and prefer to use your own hand writing rather than computer font, then you need a good sturdy pen. I also do an enormous amount of doodling. There are so many kinds on the market, it's difficult to know what to choose. Most cropping stores will let you try before you buy, so take advantage if you can.
Things to look for in good marker or pen:
The SMEAR factor: Does it smear when you touch it with your fingers?
How long does it take to dry? I'm an impatient scrapper and end up ruining things because I can't wait for slow pigments to set.
Does it run if there is dampness or water? Not that your pages should be getting wet anyway, but if you sometimes use acrylics and you want to paint overtop of your ink, you need to ensure your pen won't run.
Cost: Don't cheap out. This is an essential.
Writing Materials: Will it write on different makes of paper, vellum, cardstock, foil, photo's, ect?