I use a Papermate non stop pencil (HB) & A5 lined spiral-bound pad (though I'm thinking of upgrading to A4 non-lined... go me!).

I find pencil much smoother to write with and being able to cleanly edit my notes without losing page real estate to scribbling out is always a bonus. I also just write on the facing page; when I reach the back of the pad, I turn it over and work back to the front.

If I use a biro/fountain pen, the ink bleeds through the paper, wasting perfectly viable space on the reverse of the page!