Pens are all well and good, but what are people's favorite notebooks? I've found that the paper is perhaps just as important as the pen in a satisfactory writing experience.
I've used Leuchtturm1917 notebooks (1) as my journals for several years and recommend them; the paper is smooth and good for writing and drawing. My local art store carries them, which is a plus - I love buying local.
I've settled on the B5 10"x7" size (2) with blank pages. I love that it has two ribbons for keeping my place, and a handy pocket (I keep stamps and stickers in mine). The hardcover versions are sturdy, which is perfect because my notebook goes everywhere with me.
At first I used the ruled and or dotted versions, but now prefer blank pages for drawing.