I began with moleskins, A6 and A5. Light and transportable, they are excellent for carrying all the time. The drawback is the paper has a yellow cast and is slightly translucent, great for authenticity, but not so good for scanning.
Muji (the light brown and red-edged) make a really broad range of size and they are really thin, perfect for filling a sketchbook in a morning, so I thought I could make each one location specific. However the paper is absorbent and seems to suck up graphite and colour. It's hard to get a strong tonal contrast with them.
I tried A3 photocopy paper, which is actually excellent for scanning pencil drawings - who'd have thought, but obviously too thin for any rough treatment.
The white spiral bound book has no provenance I'm afraid, it's white paper is just a bit thicker than photocopy paper and it's great. Small enough to take out for serious drawing, it lays flat and scans beautifully.
The very best sketchbooks I have found, I bought in the London Graphic Centre (you can get them online too). These are Leichturm1917 series. Flat, opaque white, smooth paper and the right thickness. I think they are just about perfect - I wish they made spiral bound versions.