What material is the smoothest for air to travel over the wing?
Well first let's dispel the rumor that wings are "pretty smooth" - ideally (in the dreams of aircraft designers and the mathematical perfection of fluid dynamics modeling) they would be, and they certainly are from a distance, but when you get up close and personal most aircraft wings are ugly, bumpy things (as you'll see below). Modern composite wings are a notable exception to this general rule, but there are a few other exceptions in the history of aircraft design.



The material is, for the most part, not relevant: We usually paint aircraft, so the smoothness of the underlying material contributes less to the surface than the paint preparation, clear coat, and wax.

What does matter are the construction techniques used. A few examples (in rough order of decreasing smoothness):  


A modern composite wing can have an almost completely smooth surface (with no rivets or seams in the airflow) - for example a 787 wing, or this SR-20 wing:

Some aluminum designs use "bonded construction" (glue) with fewer disruptions of the wing surface. One example are the Grumman AA-1 and AA-5 series aircraft, which has a smooth wing except for the fuel caps and contoured seam lines:

Conventional aluminum wings can be assembled with flush rivets, which once primed and painted give a very smooth surface with only slight irregularities around each rivet:

(Please ignore the blue protective film, and open rivet holes. This is from an RV-14 under construction &amp; was the best photo I could find.)
Fabric wings - one of the oldest wing coverings - are actually pretty smooth. They have "rib stitching" to attach them to the substructure, and are stretched taught over the frame (using heat &amp; dope to shrink the fabric), but the overall result is a very smooth surface between the ribs, with some irregularity over them from the stitches:

Typical aluminum aircraft wing is attached with typical rivets - the head of which protrudes into the airflow slightly. You can see some rivet (and screw) shadows in this image:





Note that I've only considered the TOP surface of the wing (where airflow and smoothness is most critical). The underside of a wing usually has many other things sticking out into the airflow (fuel tank vents, access/inspection panels, hinges &amp; tracks for control surfaces, etc.) which are a much greater disturbance to the airflow than whatever fasteners are attaching the skin to the rest of the structure of the aircraft.