I like good paper in a sketchbook. The problem is that on different days my definition of a good paper is different :) Sometimes I want real high-quality watercolor paper with texture and character, other days I want a smooth surface, and sometimes all I want is something that is slightly coated and will let my pen glide.
Often my favorite paper is the one that is within reach when I have a chance to draw. It is impossible to have everything in one book so I solve this by taping, gluing, or sticking all sorts of papers in my main sketchbook. Very often it is Stillman & Birn Alpha or Strathmore 500 mixed media journals.
My sketchbooks are filled with experiments. And that is a reason why I think it is important to not know what kinds of things will be happening in a sketchbook before I pick one. But sometimes it is fun to try and list my plans - even if only to later see them burn :) I just finished Strathmore sketchbook - and this is the back cover where I tried to puzzle out what kinds of things I will be making in Sketchbook #139.