What is the ideal paper for drawing a sketch (portrait)?

Sketch paper is meant for large, quick sketches in dry media, without much detail. They aren't usually meant to be permanent. So sketch paper is usually wood-pulp based, thin and cheap, and sold in large, thick pads. If you want your sketches to last, choose acid-free sketch paper, and if you prefer thicker paper, choose a heavier weight - at least 125gsm/80lb. Expressive sketching calls for paper with a slightly rougher texture. Generic sketchbook paper has a fibrous texture that allows tiny specks of white to show through the shading and a toothy surface that will hold onto the softest of mediums. Try the 80lb Blick Drawing Pads or Canson Heavyweight Sketch. For sketches with a little more detail, a better quality sketch paper will give you a finer surface to work on. Strathmore Windpower Sketch is a lightweight (almost transparent) paper with a fine smooth surface, while Windpower Drawing is a little heavier. The surface of sketch paper shouldn't detract from a sketch, but it often doesn't really add anything, either. If you want a stronger texture through your sketch, consider a medium-surface drawing paper such as Lana Dessin. The double sizing gives a firm drawing surface while the even grain gives sketches a distinctive look.