I choose unpainted titanium, because after sixteen years of parking dings, crashes on ice, and encounters with gravel it still looks good (scratches and small polished spots from such things are the same color as the frame and stay that way in spite of exposure to road salt and rain) while my last painted steel frame picked up all sorts of primer and rust colored spots that contrasted and looked bad in half that time.

I'm also open to stainless steel which should work as well for the same reason.

I'll probably use a pair of Velocity rims with the retro-reflective (grey during the day) Halo coating in the next wheelset I build. Bright lights are enough to get noticed on both ends although something on the side seems like a fine idea.