A nice brown Barenia calfskin leather strap would look nice, like the one that came on the First Omega in Space edition. Similar case and dial, just different hands. You can still purchase it from Omega (for about $260 without a clasp) but you can find other options online for less. I purchased one that was a lighter brown from Milano straps:
I would say leather. My favorite right now is the black shark skin double ribbed with curved ends (but of course this was not made for 19mm lug width). So my search for an inspiring strap continues...