The eye can perceive a very wide angle, but of course much of what your eye sees is not in focus. At any given time, only light falling on the center few degrees of your field of vision (your fovea, packed with color sensitive cones) is in the so called "sweet spot" of vision.
So, going with that concept, the lens that approximates your eyes' "sweet spot" can range from 80mm to over 100mm. Think about it: you may be able to "see" a wide field, but you cannot keep it all in focus at one time.
Many places will also say a 50mm lens (~32mm on APS-C) is a "normal" lens since it approximates what the eye can see. I haven't found this true in my experience, and I believe it is more attributed to the fact that a 50mm lens has almost no perspective distortion since 50mm is the diagonal of the frame size of 35mm film.